You have the extra pawn, but it has not crossed the middle of the board, and the black rook faces it from c8. The winning plan is king two squares diagonally from the pawn, then the rook guards the pawn, then the pawn advances. The rule: when the pawn is short of the mid-line and the enemy rook stands in front of it, three ranks away, every step of that plan gets stopped, and the game is a draw.