In a fortress a piece or pieces are unable to break into the enemy's camp. They are most often seen in the ending because they involve keeping the short-stepping King out of the enemy position. Hence, fortresses are generally used to hold the draw. Fortresses can render an otherwise winning material advantage useless. In a recent correspondence game I unwisely played the Modern Defense as black. I say unwisely because even with engines it's well known that for some reason engines do not handle K-Indian type formations well. And as early as move 16 the engines were showing white as having about a one P advantage and things only got worse. In the following position Stockfish evaluated the position at a little over a two Pawn advantage in white's favor.
It's obvious that black is about to lose his Q after 43...Ra7 44.Rxh7+ Qxh7 45.Rxh7+ Rxh7 leaving white with a Q against a R and B. But, I realized there was the possibility of setting up a fortress and after 52 moves we reached the following position which the engine was evaluating at almost 4 Pawns in white's favor. Normally that would be an advantage that was more than enough to warrant resignation, but not here.
Multiple Shootouts with various engines didn't show how white could win and at move 64.Kc6, white offered a draw because there is simply no way he can make progress. Shootouts with Stockfish and Komodo all went over 100 moves and at 29 plies the following position was reached after 103 moves. Notice how black's pieces all protect each other and white has no sacrifices available with which to break through.
White's h-Pawn can't queen and black's f-Pawn is a real threat. As a result, white has nothing better than a perpetual check. Black only need be careful that his R doesn't vacate the h-file when white is in a position to sacrifice his Q for the B. In that case he would have a passed P on h7 and d5 which would prove more than the R could handle!
Fortresses are a handy thing to know and Chess.com has several articles on them that are worth looking at.