Keyframes record the beginning and end of each transformation of an object or element in the scene. The values at these keyframes are called keys.
For example, if you have a box that has not been animated, no keyframes (or keys) exist for it. If you turn on the Animate button, move to frame 20, and rotate the box 90 degrees, Rotate keys are created at frames 0 and 20.
The key at frame 0 represents the orientation of the box before it was rotated, while the key at frame 20 represents the orientation of the box after it was rotated 90 degrees. When you play the animation, the box rotates from 0 to 90 degrees over 20 frames.