what is a keyframe

A key frame is a full frame recorded in the video. Most of the time compressed videos do not store the full image at each video frame, as this requires a lot of space. Instead, they will record the change between one frame an anothe, and will attempt to use this data to build the video.

You can see this effect as watching a video in reverse is much slower than watching it forwards for many video formats, as to build each frame, the video must start at the key frame. Most video players will simply just show keyframes instead.

You can also see this during tramission errors on digital television, when things seem to "ghost" or "block" until it magically fixes itself (new keyframe).

This is an over-simplification, but is on the right track. Basically more keyframes mean better video quality and resilience to errors, but at a large data cost.