Animation has always been about the illusion of movement. Tweening is a major part of making that illusion look real. Tweening in animation is a short for inbetweening and it’s the process of generating images that go between keyframes.
Keyframes are the images at the beginning and end of a smooth transition. For example, an animated character could appear to jump from one point to another. That character would be clearly defined in each keyframe, but between those two frames, the figure could look distorted or stretched to fit with the motion and direction.