Onion skinning is a 2D animation technique. In the making of traditional animations, artists draw on very thin papers and place the papers on a light source. By doing so, artist can see through the frames, and compare the keyframe and the inbetween frames.
In Animation Desk, you can open the onion skinning feature in the settings. Be sure you also turn on the lightbox. There are different options to see through the frames.
Left: Onion skin off
Right: Onion skin on