1… Qa7 is a skewer on white’s queen that wins the white rook.
The point is that white can’t capture the black queen, Qxa7, else Bxc3# would be checkmate.
White also can’t defend their queen in any way (the knight on c3 is pinned). And if the queen moves away to a safe square, say Qc4, then black can then capture the undefended rook, Qxh1.
Note that 1… Qb6? fails because after 2.Qxb6 Bxc6+, white can block the check with Qb2 and still be ahead in material after Bxb2+ Kxb2.