Lesson
White to move. Main line: 1. Kf6 Kf8 2. e5 Ke8 3. e6
Kf6 places the white king on a key square for the pawn on e4. From there the king controls e7 and f7 and can make progress even when Black heads toward the promotion file.
White advances only after improving the king. Following 1...Kf8 2.e5 Ke8 3.e6, the pawn has reached the sixth rank with its king in front and Black cannot hold the necessary opposition forever.
For a non-rook pawn below the fifth rank, the three key squares sit two ranks ahead of it. Reaching one with the king guarantees promotion against a lone king. This geometric rule is more reliable than memorizing a single move order because it tells you what the moves are trying to achieve.