What is a keyframe?

A. It's the frame of the animation that defines the meaning of the action.
B. It's a frame in which resides all the weight of the whole scene.
C. It's the stored transform status of an object in a given frame.
D. Keyframes don't exist because every frame is equally important for an animation.

C – A keyframe is the saved status of the transforms of an object or rig at a given frame.
Having different keyframes in different frames is what makes objects and characters move.