What is a fortress in chess? Throughout the 2016 World Championship match, analysts have made many references to the term "fortress". What does it mean, and how can I use it to my advantage?
A fortress in chess is a position in where the weaker side defends by making waiting moves, and where the stronger side cannot make any progress as long as the defender does not make a crazy move. A very well-known fortress position is the following:

[fen "6k1/6p1/5r1p/8/Q7/8/7P/6K1 w - - 0 1"]


It is impossible for white to make any progress if black just moves his king around in the corner, since the black rook denies the white king any possibility to infiltrate as long as it stands on f6. Since the rook is guarded by the g7 pawn, and the g7 pawn is in turn guarded by the black king, the only way to make progress would be to force the black king out of the corner. However, this is not possible.

The way you use it to your advantage in your own games is as always: understand the concept, and recognize when a fortress is possible in your own games.