In baseball, the act of observing, filming, or reviewing footage of signs used by opposing teams and deciphering them in order to gain a strategic or tactical advantage during a ballgame.
Stealing signs is not always against the rules. Major League baseball's prohibition pertains to stealing signs via electronic means only. Attempts by a team to steal signs during the game simply by observing or by other allowable means are permitted, but obviously risky and unpopular. There are a number of unwritten rules in baseball, one of them being that teams act in a sportsmanlike manner, inferring that sign stealing is a breach of etiquette. In reality it is a baseball tradition to gain the advantage in a game by any available means-and there has certainly been evidence of sign stealing when not strictly against the rules.
With the advent of live camera footage and large scoreboard screens, teams have done their best to keep signs hidden, whenever possible. The most obvious example of this is when players meet on the field and cover their mouths with their gloves to prevent lip-reading by anyone observing them. While sign stealing seems a serious offense, most players and managers tend to take it lightly (as long as official rules are not breached), as it's all "part of the game."