I guess the title gives away the answer here, but white has just threatened black’s bishop.
How does black reply?
The answer surprised me, and even now that I know the answer, I don’t really see it. I suppose I need to play out the various combinations several times, and even then, I don’t think that I am a good enough chess player for a move like this to ever become second nature to me.
The solution is that black just ignores the threat on the bishop and pushes the pawn. If the bishop is taken, then he pushes the pawn a second time. That create back rank mate threats which adequately compensate for the loss of the bishop.