Crazyhouse chess is a chess variant similar to Bughouse chess but played with 2 players and no teams. Each player has a "pocked" or "reserve" for pieces. Every time a piece is captured the capturing player gets a piece of the same type and of their colour in their reserve. At any time that player can drop that piece on any vacant square on the board, but NOT to make check. The same rules as regular chess apply except for some new rules. The first rule is that pawns can't be dropped on the 1st and 8th ranks. The second is that pawns that get promoted and then captured are dropped back on the board as pawns. Third, unlike in Shogi, a Japanese chess-like game that is similar to crazyhouse chess, you can drop pieces that result in immediate checkmate. Lastly, in crazyhouse the usual algebraic notation is extended to include the dropping of pieces. For example, "Q@h8" means "Queen is place on h8 from reserve".
For example lets assume that in this position white has a queen in their reserve.
Next, white drops a queen for checkmate.
This is just one example of a possible endgame scenario in crazyhouse chess.