It’s been a powerful few years for labor. From the fast food workers’ fight for a $15 per hour minimum wage to the mass teachers’ strikes against education budget cuts to professional athletes taking a stand against police brutality by refusing to play, workers across industries are fighting back against growing instability and inequality.

These inspiring acts of mass solidarity show us what can happen when workers come together to demand fair conditions and an end to the oppression of themselves and others. But we should backtrack a little. Labor terminology can be confusing, and you’re probably not learning much about it in school, either. But as the class struggle continues to boil over around us, it’s important to know the difference between, say, a “boycott” and a “wildcat strike.” What exactly is “crossing the picket line?” What does it mean to “go out on strike” or to be a “scab”?

One particularly instructive example of how strikes and picket lines operate came — and how unionized workers don’t always support each other’s interests — came in the fall of 2018. At the time, over 7,700 hospitality workers went on strike in a massive nationwide hotel work stoppage. But multiple unionized professional sports teams — from the New York Yankees to the Philadelphia Flyers ice hockey team — violated a cardinal rule of the labor movement by crossing the hotel workers’ picket line to stay at local Ritz-Carlton hotels while traveling for games. Many of the teams were lambasted online for their lack of solidarity with the workers there, most of whom were women, people of color, and immigrants.

Simply put, a strike is a planned work stoppage that occurs when the members of a union collectively agree to refuse to work until their demands are met. This usually happens after contract negotiations with management have broken down and a majority of members have voted to authorize the strike, which is what the Marriott workers had done.

The teams who crossed the picket lines have incredibly strong unions backing them and enjoy the benefits of said unions’ lucrative collective bargaining agreements. Thanks to the players’ union, the Yankees and the 29 other Major League Baseball teams covered are among the highest-paid athletes in the world, with players’ paychecks running well into the millions.

Meanwhile, the average hourly wage for a hotel housekeeper in America is $11.10. After contract negotiations fell apart, the striking Marriott workers voted to risk their livelihoods to go on strike for improved healthcare, higher wages, and stronger sexual harassment protections. These marginalized employers took a huge risk to try to force a change for the better. Whenever this happens, union members across all industries owe it to them to support their efforts, no matter how different those industries may seem. For a union member to cross a picket line — as these sports teams did — is a sign that they either do not care about the strike or have actively decided to go against it. Either way, they demonstrated that they have sided with the management over the workers who are on strike.

Historically, companies whose workers are on strike have hired outside workers — also known as “strikebreakers” or “scabs” — to take their places, and have sometimes resorted to violence to break strikes, as seen in West Virginia’s bloody mine wars that stretched from ‎1912 to 1921, or New York City’s Uprising of the 20,000 in 1909 where multiple women garment workers, including union leader Clara Lemlich Shavelson, were brutally beaten by company-hired strikebreakers.