When you consider an attacking sequence, you want to see all the pieces that you and your opponent have bearing on it, including pieces aimed in the right direction with or without obstructions in their way. Here that means seeing not only the obvious back rank mating idea for White but also the Black bishop on c5 and the White bishop on a3. The Black bishop tells you that while the battery of rooks on the e-file looks formidable, it isn’t quite enough to mate by itself: while Black only protects the e8 square once, he has an interposition with Bf8. Yet White’s bishop on a3 makes the idea work after all, since it permits his rook to capture on f8 with impunity. Thus 1. Re8+, NxR; 2. RxN+, Bf8; 3. RxB#: the White bishop serves to prevent a recapture by Black’s king. As you look at the original position here, notice the way White’s bishop attacks f8 through the Black bishop that stands in its way. This is known as an “x-ray.”