"tweening" is short for inbetweening, which is the process of drawing frames in between keyframes (Frames containing the key poses of a scene). There is a process know as "motion tweening" In Adobe flas, in which you create an object and have it in two keyframes, then you are able to put frames in between and use the program to generate frames in between the two keyframes. Motion tweening in not available on flipanim, and regular tweening is just a key feature of animation.