A forced mate is when one side is driven into a checkmate because they have little or no choice in the moves they can make. Look at todays puzzle which is a forced mate in six moves. Black has little or no choice in moves to make and every move leads to being checkmated. That's called a "forced mate" or white can "force mate" in six moves or less. In other words, white forced black into a checkmate.
http://www.chess.com/forum/view/daily-puzzles/6112009---mate-in-6
What is it ?