What is a key-frame?

Keyframes are special frames used in compressed video formats which tell the codec useful information about the frames coming after them. This allows the frames in between keyframes to be compressed into a much smaller space than if they each were a full description of the whole image. Often the in-between frame information can just be a list of the differences between the new frame and the previous one.

For files designed for maximum compression, the amount of keyframes can be kept to a minimum, and a keyframe might only be used when the entire scene changes between one frame and the next, thus requiring a keyframe to describe fully the frames to come. This can present difficulties when a user wants to jump to different part of the video; the player might have to spend a long time finding the last keyframe, then decoding all the frames from there to the desired spot in the video.