White’s move threatens to win the pinned black rook on d7. (White now has 3 attackers on the rook, black has only 2 defenders.)
The only way for black to give extra defense to the rook, Qe7, will leave the c6-square unprotected – which allows Qc6+, followed by Bxd7.
Note: 1. Qd3 is not as good because black simply plays Kc7! Keep in mind that white is already down in material, so exchanging his bishop for a rook would still leave him in a bad situation.