Basically it is the part that holds the axel stubs in. Most of you wont have this on the rear differential/axle but all of us have it in the front. Mine is specifially from a rear LSD.
Not sure how long the image will last but
The part is the diameter and thickness of a quarter aproximatly. It is cylindrical and made of metal. I broke one. Its from the rear differential but I know these are a super generic part. I found a bunch of places online that carry every random size of clip imaginable. Problem is that the few I contacted wanted orders of 50 units or some other insane amount.