The catcher needs to know what sort of pitch the pitcher will throw so he can set up correctly to catch the pitch. To make sure catcher and pitcher are on the s

The catcher needs to know what sort of pitch the pitcher will throw so he can set up correctly to catch the pitch. To make sure catcher and pitcher are on the same page, each pitch starts with the catcher giving hand signals to the pitcher to suggest he throws a specific type of pitch. These hand signals are what is meant by "signs." An opposing team can "steal signs" by looking at the catcher while he's giving his signs and then somehow relay that information to the batter. That way the batter knows what type of pitch is coming. Usually the information is given to the batter by some other type of signal, that's what "banging on the garbage can" was, a signal that a specific type of pitch was coming. (In this case they were signaling if a pitch was going to be an offspeed pitch) So basically what happened was Astros set up a camera in the outfield aimed at the opposing team's catcher. The catcher would signal for, for example, a breaking ball. Astros employee viewing video feed would see this and then bang on the garbage can to indicate to the batter that the pitch would be offspeed. It's important to know that "stealing signs" the old fashioned way (without cameras/technology) is legal and part of the game. This is why catchers will give multiple signs when runners are on base, because base runners may be able to look at the catcher while he's giving his signs and relay information to the batter. When the catcher is using multiple signs there's an agreement beforehand between the catcher and the pitcher which of the multiple signs is the "real" sign. There's a Wikipedia page on sign stealing: https://en.wikipedia.org/wiki/Sign_stealing https://en.wikipedia.org/wiki/Sign_stealing