Onion skinning
Showing translucent ghost images of the neighbouring frames at the same time, so the animator can see the motion between them.
The name comes from traditional cel animation, where thin translucent paper on a lightbox let the artist see the adjacent drawings through the sheet. Digital tools — sprite editors like Aseprite, timeline editors, 2D rigs — do the same by rendering neighbouring frames tinted and semi-transparent, usually the past in one hue and the future in another.
The point is to judge spacing, timing and arcs of motion without flipping back and forth. It is standard in frame-by-frame animation and flipbooks, where every image is drawn by hand and the rhythm between them is the whole craft.