Explanation

The quickest checkmate in the game of chess is possible after the opening moves:
1.f3 e5 2.g4

And after only two moves, the game ends with Qh4#.
As this happens only after White playing the worst possible moves, this opening is called Fool's mate.
The checkmate pattern is named accordingly.

Examples

White can deliver Fool's mate with Qc3#.
The rook on g8 can't block the check, because it is pinned by White's rook on f8.

Black can deliver discovered Fool's mate with Rc3#.
The rook discovers the mate by the queen, and blocks the attack from White's queen on Black's queen.