After pawn to b5pawn to b5 the shrinks to b5-e5-e8-b8 and the black king on g5 can no longer enter it: the pawn on its own. Any king move lets Black step into the square and draw.
Moves
No moves yet.
About this endgame
Same board, but now it is White to move. Here the works from the other side: it tells you whether your pawn can run on its own. Every time the pawn advances, the square shrinks by one rank and one file, and the enemy king has to be inside it after its move to catch the pawn. That is why bringing your king makes no sense: it stands at the far end of the board and, meanwhile, the black king steps into the square. This is a typical situation in pawn endings with a far from the kings. An drags the enemy king towards it, and sometimes the king does not even get there. Before trading the last pieces into a pawn ending, look at the square: it often tells you whether that ending is won without calculating a single move.