Sign stealing is the act of players and coaches on the field, watching the codes the catcher gives the pitcher, and relaying it between players via hand-signals and gestures to tell the batter what kind of pitch to expect.

In modern days, this even includes watching video-replay to capture the signs, and then counter-investigations identifying the sign-stealers and their methods.

Turns out there's a whole host of in-the-open sign language happening during stickball games.

https://en.wikipedia.org/wiki/Sign_stealing