This is a phrase used in baseball to describe an illegal action in which the pitcher pitches the ball before the batter is prepared. In Major League Baseball, if an umpire believes a quick pitch has occurred, they will immediately call a balk or call the pitch a ball. A balk is called if there are runners on base; a ball is called if there are none. This action is a violation at all levels of competition. This rarely occurs at the Major League level, when it does it is typically a mistake on the part of the pitcher.
A quick pitch is a violation at all levels of play because it is a dangerous play and goes against the spirit of the game. If a batter is not prepared to receive the pitch, they do not have a chance to defend themselves should the ball not travel to its intended target, or they may be inadvertently standing in the path of the pitch. It goes against the spirit of the game because at the core of the game is a pitcher and batter. If the batter is not ready, there is no real competition, only a pitcher taking advantage.