Any action in combat that may not directly result in damage but some other form of debilitation or advantage that allies can capitalize on.
Combat maneuvers typically give some field advantage of some sort if they are successful. They generally do not cause direct damage but merely tip the scales of combat.
Common examples include:
Sunder- damage or destroy an item
Steal- take an item from an opponent with subterfuge
Grapple- limit the actions an opponent can take