What makes it strong
Material is level: two pawns each, and both sides have a passed pawn. Black's e-pawn is in the centre, close to both kings. White's a-pawn is on the edge of the board, far from everything else. That is an outside passed pawn.
When the outside pawn advances, the defending king has to go after it. The further it travels, the further it is from the other pawns, and the attacking king is free to win them.
The plan
First stop the enemy passed pawn with your king. Ke4 blockades the e-pawn here. Then push the outside pawn as a decoy. When the black king goes after it, take the pawns it has left behind.
In the demonstration line, by the time Black captures on a5, White has won e5 and g5, and the g-pawn promotes. The tablebase gives a win whatever White plays first, and Ke4 is the fastest.
Typical mistakes
Do not swap the outside pawn off or let it be won for nothing. Its value is the time the enemy king needs to stop it, so keep your own king near the other pawns, ready to collect them.
The same idea explains why a pawn majority on the side away from the kings is often an advantage in pawn endings: it can produce an outside passed pawn.
Remember
- Far from the kings is best
- Use it as a decoy
- Blockade the enemy passed pawn
- Collect what is left behind