Opposition
Opposition is a relationship of the two kings, one square apart on a line, where the side that does not have to move can keep the other king out.
Direct opposition is this picture: kings on the same file with one square between them. Distant opposition is the same idea with three or five squares between. Outflanking is how the king that holds the opposition walks around.
White to move must step aside. Black then keeps the opposition and can shoulder White away from the key squares in front of the e-pawn.
Most king-and-pawn endings are opposition plus zugzwang. If you take the opposition on the right rank, the pawn promotes; if you lose it, the game is a draw.