White’s queen is aimed at Black’s king and masked only by his pawn on e6. If the pawn could move forward with a threat, White would have an effective discovered check. The e6 pawn can’t move forward because Black’s bishop is in the way; and if the pawn were able to move forward it would have no target (promoting on e8 isn't going to work because Black will have the square guarded). If those two problems are perceived clearly it also becomes clear that they can be solved at once if the bishop can be moved to d8. Try putting a piece there to force a capture on that square: Rd8+. Notice that with White’s e6 pawn attacking the f7 square, Black’s king is trapped on the back rank. Black has to play BxR, and now the way is clear for e6-e7+, with the pawn promoting via e7xB on the next move.

The idea here might as well have emerged by just looking at checks. White’s only one is Rd8, and Black’s only defense is BxR. If you imagine the resulting position clearly, the discovered check for White is obvious.