Judy Woodruff:

Today, the president retweeted some of this video.

Also in the crowd last night, people who believe in the QAnon conspiracy theory. QAnon claims that a shadowy cabal within the U.S. government is at war with President Trump, and that the president will soon purge the nation of these enemies.

For more on this conspiracy and how it's spread, I'm joined by "NewsHour"'s P.J. Tobia, he's been following this.

P.J., first of all, what is QAnon?