This term is used in baseball to describe the action of a batter attempting to see the signals the catcher is giving to the pitcher, or where the catcher is setting up to catch the pitch. The signals usually describe the pitch about to be thrown, but may also signal fielding positions, or to tell the pitcher to throw the ball to first. While this action is not strictly forbidden in Major League Baseball, it does break a moral code that exists in baseball. Many major leaguers look down upon players who do this. Some even consider it cheating.
There are many customs and traditions in baseball that are followed even though there is no rule. Not looking at the catcher while up at the plate is one of them. Another related tradition which is a bit more accepted, but not much, is for a runner on second to try and "steal" the signs from the catcher and relay them to the batter. It can be very difficult, and the team on offense will typically use multiple signals to confuse the effort.