The concepts of blockade and fortress in chess refer to techniques used to limit the mobility of the opponent's pieces and restrict their ability to make threats. A blockade is created when a player's pieces occupy squares that their opponent's pieces could potentially use to move and attack. This can limit the opponent's options and make it difficult for them to make threats. On the other hand, a fortress is a stronger form of blockade, where a player's pieces and pawns work together to completely restrict the opponent's pieces from moving.