Inbetweens are
animation drawings that the assistant
animator has left for the inbetweener to do.
These drawings are called inbetweens because they are between the drawing of one phase of an action and another phase several steps later.
As an example, a script has Mickey Mouse sitting in a chair, then standing up in front of the chair quickly. The assistant animator would draw the key frames, one of Mickey in the chair, one with him halfway out of the chair, then the final frame has Mickey standing. The inbetweener would then take these three key frames and produce the 105 to 165 inbetween frames that would make the action appear fluid and cohesive. This would make five seconds of actual film.
The inbetweener is usually a relative novice in the animation field. It is the lowest rung on the animator ladder.