Lesson
White to move. Main line: 1. Rb8#
The rook on a7 fences off the entire seventh rank, so the black king cannot step forward. Rb8 then checks along the eighth — with nowhere to go, it is mate.
This is the end position of the "ladder" (or lawnmower) technique: two rooks alternate ranks, each cutting off the row the king just left. The full ladder is in the mate sequences section.
The one thing that goes wrong is the king walking at a rook. If the black king ever steps toward the rook holding a rank, swing that rook to the far side of the board: it keeps the cut-off, stays out of reach, and the ladder continues unbroken.