In the game of pool, it is one of your balls that sits in the pocket but has not fallen, thus blocking another from putting their ball in that pocket. Good for defense. Usually you don't aim for that ball unless you have to. Also, when you do choose to take that shot on the duck, it is very easy and allows good ball movement afterwards.
"Man this guy is good. I'm gonna keep that duck in that pocket to block his shot." -or- "My duck is blocking his ball, so I'm good" "That ball in the blocking the pocket is A duck!"