What is Onion Skinning in Animation?

Onion Skinning:

While performing motion tweening or drawing a particular object, it is necessary to view its position in the previous and subsequent frames. This technique is called Onion Skinning. Onion skins are multi-layered and thin enough to see light through them.

Onion skinning allows the designer to see several frames of the artwork displayed at one time. It can be turned on and turned using the toggle button on the document window’s top.