Frame by frame refers to the process of animating by manually redrawing each frame by hand. This can be done traditionally or using digital software.
This sometimes results in a "wobbly" effect on lineart due to lineart being redrawn.
It is not mutually exclusive with motion tweening, and both are often utilized together. However, you shouldn't tag both if one is just being used for minor elements. Examples of mixing tweening and frame-by-frame
See also