From Wikipedia: "A key frame in animation and filmmaking is a drawing that defines the starting and ending points of any smooth transition. The drawings are called "frames" because their position in time is measured in frames on a strip of film. A sequence of keyframes defines which movement the viewer will see, whereas the position of the keyframes on the film, video or animation defines the timing of the movement. Because only two or three keyframes over the span of a second do not create the illusion of movement, the remaining frames are filled with inbetweens."
In other words, it is a raw still shot that was used directly for the producing the animation. This covers parts, or even all of, the draft sketch to test out plot flow, pre-color scenes, cel that was used for the animation, or complete film still. It is primarily used to complete the animation, since key frames themselves do not show any movement; it is important however, if not most important in developing animations.
It is not necessarily same thing as anime screenshot, which captures the final material in presented medium. Key frames in general strictly internal-use only.
The Japanese word 原画 (genga) can mean "key frame", but more literally means "original illustration". Thus works tagged with this on Pixiv should not be automatically tagged as key frame here.
