White has played a very poor and weakening move and allowed Black the quickest possible checkmate (Fool's Mate) in the opening game of chess: Mate in two moves. The mate almost never occurs in practice but is commonly known among chess players, due to it being the fastest possible checkmate. However, Black was already better.
A variation of this mate occurs when White advances the f-pawn two squares: 1. f4 e6 2. g4?? Qh4#. White can also move his g-pawn before the f-pawn.