[mattst88]

Signs a couple of different things: most notably they're signals from the catcher to the pitcher suggesting a pitch for him to throw, but also there can be signs from the dugout to a batter telling him to bunt, or to a runner on base telling him to steal, etc. If the other team is able to decipher any of these, they'll have a significant advantage.

To be clear: stealing signs isn't cheating. It's using electronics to steal signs that's cheating.

For example, a base runner on second can see the catcher's signs to the pitcher and relay them to the batter so he knows what the pitch is. This is not cheating.

In 2017, the Astros used electronics to steal signs. They had an employee somewhere beyond the outfield wall with a camera who watched the catcher and relayed his signs electronically to someone in the Astro's clubhouse (behind the dugout) who then banged on a trash can loud enough for the batter to hear. That was cheating. See https://www.youtube.com/watch?v=FUkJeko0QGE

To prevent that sort of thing from happening in the future, MLB last year allowed pitchers and catchers to switch to an electronic system for relaying signs (the catcher has some buttons/keys and the pitcher has an earpiece).