Chess theory is an impersonal, objective assessment of a chess position. It generally refers to known positions which have been studied or played by strong players. Although theory can apply to any phase of the game, it usually refers to the opening and endgame.
Opening theory means opening moves which have been played between strong players. The moves are considered to be the best (or equally attractive) in specific positions. 'Theory says this move is strongest' means the move is thought to lead to the best result from that position.
Endgame theory means endgame positions which are known to lead to a certain result. 'Theory says this endgame is drawn' means that with best play on both sides, neither side can win.
Middle game theory is more abstract and refers to families of positions with similar characteristics. A good example is the isolated Queen's Pawn (IQP), which theory considers to be strong in the middle game but weak in the endgame.
