Black’s bishop masks his rook on d7, and the rook is aimed at White’s queen. What can he do with the bishop that will require a time-consuming reply from White? He has no checks or captures, so he looks for threats (and of course you would want to look for threats even if there were a capture to make; the threat might be the better move). With no White pieces on dark-colored squares, the bishop’s prospects for threatening anything may seem dim. But your assessment of a position always should include an inspection of the enemy king and the pressures bearing on it—the constraints on its movement, and its exposure to checks. Black has a single check: Qxh2. By itself that move wouldn't be productive, because the queen would have no protection against KxQ. If the bishop could add pressure to the h2 square, though, the possibility of Qxh2 would become a mate threat. Is there a way for the bishop to attack h2? Yes, with Bg1. Of course White escapes mate with KxB, but mate isn't the goal; the goal is to require Black to address the threat of mate and thus create time for the point: RxQ.
Note that you want to consider all the ways White could reply to the mate threat—and especially any replies that also take the White queen out of harm’s way and thus blunt both ends of the fork. Here White could respond to Bg1 with Qh3; suddenly neither part of the fork works. But Black still wins because now the d-file is clear for him to play RxR.
This position involves an important type of threat to remember: a threat not against a piece, but against a vulnerable square next to the king. When you have a piece aimed at a square adjacent to the enemy king, aiming another piece there may create a threat of mate; and if it does, that square can become a target just as sensitive as the king itself.