What White pieces are poised to be forked by a pawn? The bishop and knight on f3 and h3 are in the classic position. Black can’t get a pawn onto g4 in one move, but he can march it there, and leave White no time to defend itself, by playing g6-g5, threatening White’s f4 bishop. The bishop runs away, and now g5-g4 wins a piece. (The Black pawn on f5 provides the necessary cover, of course.)
Notice that your opponent can thrash around a bit; after the initial pawn push, White can throw in a capture elsewhere like c4xd5. In this case Black just recaptures with his e-pawn and the forking threat is both renewed and unavoidable for White, but the point is that you always want to make sure you have considered what counterplay your opponent might be able to offer elsewhere on the board. This is especially important when the sequence you contemplate doesn’t threaten the enemy with anything more than the loss of a piece (i.e., you have no checks in the picture), because he may then be able to buy time by making similar or worse threats of his own elsewhere. In this case it’s not an issue, however, White has no effective way to derail the fork.