Hopefully someone who actually knows baseball can correct me if I get this wrong, but: In baseball, the pitcher stands in the middle of a diamond-shaped field.

Hopefully someone who actually knows baseball can correct me if I get this wrong, but: In baseball, the pitcher stands in the middle of a diamond-shaped field. They throw the ball towards the opposing team's batter, who tries to hit it. The pitcher's goal is to get the ball past the opposing batter to the catcher, who is squatting on the ground behind the batter. The catcher, having full view of all the players and being close to the batter, can opt to signal to the pitcher how best to throw the ball to get it past the batter. For example, they may raise 1 finger to throw it as fast as possible ("fastball"), 2 to make it _seem_ look a fastball but actually slow it down ("changeup"), 3 to curve the ball, etc. The catcher does this such that only the pitcher has a good view of it. This is called a "sign". It's legal for the opposing team to try to see what the catcher is signaling and inform the batter, but not to do so with electronic equipment. The Astros were using video cameras to get a close look at the catcher's signs, then banging a metal drum to inform the batter of what kind of pitch was being signaled. The article has an embedded Tweet which has a good video demonstrating all this.