Two kings can't move next to each other; therefore there is an invisible barrier created when two kings are separated by the distance of one square, either diagonally or directly face to face. When two kings are opposing each other the player not to move is said to 'have the opposition'.It is a great advantage to have the opposition, as it usually means that the opponents king can't move forwards; and when it moves backwards or sideways your king will be the first to advance forwards, which is generally a good thing.
In the diagram above the two kings are face to face and there are no pawn moves. Therefore if it is Black's move then White has the opposition. If 1...Kd6 2.Kf5 and frustratingly for Blackhe still can't an active movewith his king and has to wait with something like 2...Kc6 until White plays 3.Kxg5. If instead Black tries 1...Kf6 then White gets in the other way with 2.Kd5 and wins the c-pawn.
Now imagine if it is White's move in the diagram position. Then Black has opposition and after 1.Ke3? Ke5 Black gets his revenge in identical fashion on 2.Kd3 Kf4 etc. However, because White's king is farther up the board it happens that White can maintain the blockade with 1.Kd3! when if 1...Ke5 2.Ke3 regains the opposition. Play could then go 2...d6! (Black loses as above after 2...e6? 3.Ke4). The golden rule is that Black has to be ready to meet Ke4 with Ke6 to maintain the opposition.
In this diagram possession of the opposition is immediately decisive. Whoever has to move loses the pawn and the game.