P-Frames - the real meaning

The name is short for Predicted frames. 

These frames can refer to other frames in order to reduce the frame size.

In other words a P-frame can say "this frame is the same as the last frame except that the football player has moved and the ball is new - this is what the ball looks like"

P-Frames are significantly smaller than Key Frames, but jumping to them is harder, as one or more other frames has to be decoded in order for the P-frame to be decoded.

Specific differences

In MPEG-1,2 and 4, P-frames can only refer to a single previous Key or P-frame.

In h.264, P-frames can refer to multiple Key, P, or B-frames