Can someone who actually watches/plays/grew up with baseball explain what the deal with sign stealing is? What are the signs doing such that stealing them is apparently considered "cheating"?
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).
More abstractly, baseball has a few situations that only exist because a team can coordinate among themselves without revealing their plan to the opposing team. This leads to more mixed strategies where a greater variety of plays are enacted with some probability. Without signs, those plays wouldn't work, and the game would become more boring.
IMO I think speeding up the game will make it more exciting, and they will figure out how to signal in the time available.
Besides this coordination, signals are also used to give instructions to individuals. E.g. base stealing or bunting. However, since these plays don't require coordination among multiple players, these could be spontaneously performed at the player's discretion in the absence of signals. This could actually make a more exciting game as players exhibit more agency and create more chaos.
When a batter gets ready to bat, or when a runner is on base, they look to the 3rd base coach (or dugout coach) for a sign. The sign for the batter might be “don’t swing at the next pitch,” or “bunt the ball (a half-swing)”. The runner might look for a sign to “steal a base.”
The catcher also gives signs—to the pitcher. The new system has made it difficult to steal these signs now.
All teams try to steal the other team’s signs. But this was always done by either the players or the coaches on the field. That wasn’t considered cheating.
In recent years, technology along with staff behind the scenes were being used to actively try to steal signs and then alert players on the field. This was considered to be cheating.