The 3 basic principles that are essential to know in the ending, which are seldom, if ever, seen in the middle game or opening are: Stalemate, Zugzwang and the Opposition.
Stalemate, as you probably know, is when a player has the move and has no legal move, but is not in check. It generally only occurs in simplified positions and is thus an important strategem in the ending.
Zugzwang refers to a situation where one must move but any legal move will be bad for them and probably will cause them to lose. Again, as with stalemate, it usually only occurs in simplified ending positions.
The Opposition refers to the fact that the 2 Kings cannot occupy adjacent squares since they would be putting each other in check. In a standoff beiween the 2 Kings, which as before, usually only happens in ending positions, the King that is NOT to move has the Opposition. The King that has to move is in a sort of Zugzwang and must move away if no other moves are available.
To illustate these principles I will show you the basic situation of King versus King and Pawn, with no other pieces on the board. This is basic to understanding and success in the ending. One must know how to win with the pawn and how to draw with a lone King.
The first position show the basic drawing position. You need to use all 3 of the basic priciples to draw.
The next position shows how the player with the pawn can win. The main idea is that if he can force the other king away from the queening square he wins.
Study these positions and learn the principles involved.