A strange thing happened in Moscow today: a court convicted Alexei Navalny —a lawyer, anti-corruption blogger, and leading opposition politician — of fraud, but released him on house arrest, and thousands of outraged Russians gathered in Moscow to protest. Though Navalny is powerless to fight the charges and police returned him to house arrest immediately when he attempted to join the protesters, the moment was a reminder that this one person has gained enough significance to attract both the attention of the Kremlin, which is widely thought to have engineered the charges against him, and the support of thousands of protesters willing to brave a sub-zero Moscow night to rally in his support.

Here's what you need to know about who Navalny is, why he has inspired thousands of people to take to the streets, and what that means for Russia.

Navalny is what Putin fears most in Russian politics

Alexei Navalny, age 38, represents Russian President Vladimir Putin's greatest political fear: an opposition leader with real, grassroots support among the Russian middle class.

Since coming to power in 2000, Putin and his supporters in the Kremlin have deliberately silenced political opposition and centralized Putin's power. Although opposition figures have periodically surfaced, they have for the most part been prominent individuals who'd found success in other arenas, such as oligarch-turned-activist Mikhail Khodorkovsky or chess-champion-turned-activist Garry Kasparov. They were essentially one-man operations — dissidents, rather than true political opposition.

Navalny is different. Although he is a charismatic figure, he has been building a base of political support for more than a decade, and his movement has proven remarkably resilient in the face of the Kremlin's attacks on him and his associates. And that makes him, in Putin's eyes, a much more substantial threat.

That grassroots base comes from Navalny's years of political activism, particularly in campaigning against corruption, which makes him both popular among many middle-class Russians and a threat to the corruption-bolstered system that keeps Putin in power.

Why Navalny's anti-corruption campaign is about more than corruption

Navalny, a corporate lawyer who lives in Moscow, has been involved in Russian politics and activism for most of his adult life. He began working with Yabloko, a liberal political party, in the late '90s. In 2005 he co-founded a political youth movement called Da! ("Yes!") with Maria Gaidar, the daughter of former prime minister Yegor Gaidar.

In 2007, he began the anti-corruption campaign that has become his signature issue. He bought small numbers of shares in major Russian companies, including state-run energy giants Gazprom and Rosneft and energy-transport company Transneft, used his shareholder status (and his legal skills) to obtain information about their finances, and then published evidence of their corruption on his LiveJournal blog, including copies of incriminating documents.

His allegations proved explosive. There was a general awareness in Russia that corruption was pervasive in Russian business, but uncovering or prosecuting the specifics of those dealings was seen as a perilous activity.

Navalny's blog turned that perception on its head. His work replaced general presumptions of corruption with specific, outrageous accusations. His documentation of graft and theft made the officials involved appear clumsy and foolish, not intimidating. And by exhorting his followers to engage in similar activism, Navalny began to normalize anti-corruption campaigning, turning it into something that ordinary people do in their spare time, rather than the futile quest of lone idealists with high tolerance for risk.

This was liberating for Russians who had come to accept corruption and intimidation as a fact of life, but it was also a threat to the powerful web of business and political interests that dominate the Russian system.

The core of Navalny's success: focus on participation rather than politics

Navalny has smartly focused his activism on the mechanics of politics and governance, which are unifying issues, rather than the specifics of issue-based politics, which are potentially divisive. (Especially so in his case, as Navalny's actual politics appear to be disturbingly ethno-nationalist and on the rightward end of the economic spectrum.)

Da!, his youth movement, organized active political debates at a time when genuine opposition was missing from state-controlled media. His anti-corruption campaign is a savvy platform from which to undermine the legitimacy of Putin's government, because its core demand is that politicians and their cronies should follow existing law, rather than a demand that the law should be changed or updated. And his broader political message is that inclusion is a defense against tyranny because "they cannot arrest us all."

That strategy has proven to be quite effective. Navalny's blog, along with his personal charisma and charming blond family, have fueled his popularity, particularly among young, urban, middle-class Russians. In 2011, he wrote a blog post calling for demonstrations to protest electoral fraud, and the protests that followed (which, it should be noted, had other organizers in addition to Navalny) coalesced into the "white ribbon" opposition movement that called for an end to Putin's regime in the Kremlin.

In 2013, Navalny ran for mayor of Moscow and captured 27 percent of the vote, finishing second place behind Putin appointee Sergei Sobyanin. Several months later, he was placed under house arrest on fraud and embezzlement charges, for which he was convicted today.

The Kremlin has no idea how to solve a problem like Navalny

From the beginning, the Kremlin has seemed clueless about what to do about Navalny.

The usual tactics that it uses to silence or undermine political opponents have proven to be ineffective when it comes to Navalny. He is not reliant on state media for attention or publicity because he has his own blog, so keeping him off the airwaves doesn't silence him. He isn't wealthy or involved in big business, so he has no dubious fortune that can be used to discredit him. And he has refused to be driven into exile, saying that he misses Russia's "black bread" when away from home.

Putin himself appears to view Navalny as a sort of political Voldemort. As Julia Ioffe wrote in 2013, the president "is too scared to utter his name out loud, for fear of legitimizing him and, like some scene out of a fantasy novel, bringing his mysterious internet power to life."

The Kremlin's main effort to oppose Navalny has been to target him with trumped-up criminal charges. He has been convicted of embezzling funds from a state-owned regional timber company, but that deal was investigated at the time and determined to be legal. He has been accused of stealing from a defunct political party, but a senior party official has stated publicly that no theft ever took place.

And today, Navalny and his brother Oleg — a former postal-service employee who is not involved in politics — have been convicted of embezzling funds by inflating shipping rates, even though the postal service itself stated that it had no involvement in the charges and no claims against Oleg.

The prosecutions have thus far failed to intimidate Navalny, and the government appears to be afraid that using harsher tactics would risk turning him into a martyr. His five-year sentence in the timber case was suspended after large protests were held, and he was given another suspended sentence today.

However, today's sentence seems to be a clear attempt to silence Navalny by threatening his family. Oleg Navalny was given a three-and-a-half-year custodial sentence, not a suspended one. NYU professor Marc Galeotti points out that Oleg's more serious sentence "is hard to interpret as anything but an attempt to use him as a hostage," given that prosecutors had actually requested a lighter punishment for him.

This could set off more protests in a very uncertain moment in Russian politics

There was a large demonstration in Moscow today to protest Navalny's conviction, but it is not yet clear what its broader significance will be, if any. His supporters have been organizing large protests for years, and they have not yet had any significant effect on Putin's strength or popularity. However, this comes at a time of uncertainty and instability in Russia due to its crashing economy — and it's clear that the Kremlin is worried.

Falling oil prices and Western sanctions in response to Russia's Ukraine invasion have caused a massive fall in the value of the ruble in recent weeks, leading to inflation and panic-buying as Russians have tried to convert the currency into more stable assets. While Putin's popularity remains very high, economic instability makes it harder to predict what the result of any protests would be.

It is clear that the Kremlin is, at the very least, concerned. The Navalny verdict was due to be announced next month, but the government announced on Monday that it would come today, apparently hoping to sap the energy of protests that were scheduled for January 15th. That strategy does not seem to have been effective: there were protests on the day of the verdict despite the short notice and bitter cold, and the January 15th demonstrations have not been cancelled. But as Galeotti points out, the Kremlin may have miscalculated in its efforts to suppress protests: this allows the opposition to show its strength by quickly mobilizing an immediate protest today and following it with the larger one in two weeks' time.

The most worrying possibility, however, is that Putin will conclude that the pro-Navalny protests are a Western plot to destabilize his government, as he appears to have believed about the Euromaidan protests in Ukraine last year. In the past, the Kremlin has attempted to paint Navalny as a foreign-funded plant. That seems primarily to have been an effort to discredit him, but if the Kremlin actually determines that the protests are a foreign-government-sponsored attempt to bring down the government, it could decide its only choice is to respond with violence.