When you move a piece, sometimes an attack on a different piece is uncovered. This is called a discovered attack. For example, on the left, White sees the chance to initiate a discovered attack, and he plays Bxh7+.
Black isn't worried, however, because he can simply take the Bishop with Kxh7. But White's move did something else. Do you see it? It cleared the path for his Queen. The Black Queen is undefended! White can then play Qxd4, a winning move.