With blocked rook pawns and an extra passed pawn, White wins if the black king, after eventually taking the passed pawn, cannot get back to stop the a-pawn; Bähr's border line tells you in advance.
This structure, blocked rook pawns (a4 against a5), an extra white pawn, the white king beside it and the black king blockading it, is the subject of Bähr's rule (Walter Bähr, 1936). It answers one question: can White win by using his king to attack a5 while the black king is tied to the passed pawn?