Inbetweening is the sequencing of interpolated frames between two endpoints.
In any animation, a finite number of main poses can be drawn to outline the flow and movement of the character. Because normal animation happens at a framerate of twenty-four (24) frames per second (24Hz), the drawn outline must be completed in order to have a plausible animation. The additional frames are called the inbetweens, because they fill in-between the poses. As a rule of thumb, the fewer the inbetweens, the faster the animation (and vice versa).