When the enemy king is near but not yet in front of the pawn, the attacking king must lead the pawn forward and keep it shielded.
White's king is on c1 with the pawn on b2; Black's king on e5 is already central. A natural reaction is to push the pawn, but every pawn move here draws: 1.b4? Kd5 (or Kd6) and the black king arrives on the b-file in front of the pawn with the opposition; 1.b3? Kd4 2.Kb2 Kc5 and the same picture.