Opposition
The opposition is a king standoff: the two kings face each other with one square between them, and the player who does NOT have to move usually wins it, because the other king must step aside. It's the key idea for walking a pawn home in king-and-pawn endings.
The kings stand face to face with one square between them and Black to move. Black does not have the opposition, so the king must give way — the side NOT to move holds the opposition.
Source for further study: Dvoretsky, Endgame Manual, ch. 1.