Calling is the rule in games where players announce the pockets where they plan to sink balls, letting an opponent or referee know their intentions beforehand. For example, 3-ball in the side pocket is a call (announcement) of ball and intended target pocket.
Modern rules do not force the player to call obvious shots, that is, balls very close to the pocket, straight onto a pocket, etc. And, even without this call shot rule, arrogant players are happy to announce to everyone within earshot where they are about to shoot the winning next ball!
Several times, players have lost tournaments remaining silent when they should have announced a call shot