Back-rank Mate
A back-rank mate is a checkmate on the very back row, where the king is trapped behind its own pawns with nowhere to run. A rook or queen slides in to finish it. Giving your king a little escape square ahead of time prevents it.
Ra8 is checkmate: the black king is walled in by its own f7, g7 and h7 pawns and has no escape on the back rank.
Source for further study: Polgar, Chess in 5334 Positions.