The opposition is a must-know rule in pawn endings.
You are said to have the opposition when your king and the opponent's king stand 2 squares apart in a pure pawn ending, and your opponent has to move his king, which allows you to gain space.
Here is an example:
Note that in our original position, all the squares in front of both kings were controlled--neither king could go there. As soon as one king stepped aside, the opponent's king could make inroads into the position.
This is especially useful in pawn endings. How do we win with King + Pawn vs. King? h- and a- pawns are a draw (black has certain stalemating ideas--but this is going off the topic of this article), but how do we win with the other pawns?
Well, we gain the opposition.
Now here's a little test:
If you were black to play in this position, how would you draw?
And what about here:
Hopefully you now understand the concept of opposition and how to employ it in your games.