A forced move is a situation where the best move (the forced move) is clearly better than the second best move. For example, after 1.e4 e5 2.Nf3 Nc6 3.Bb5 d6 4.Bxc6+, black is forced to play bxc6, since other moves are clearly weaker. Another meaning is that the move one side must block or capture a checking piece, as their king cannot be moved our of the way.
