lalunette
+1y
It is a C-shaped pin/clasp (whatever you want to call it) that goes around the base of the handle and clamps onto the shaft of the window crank coming out of the door.
There is a special tool (costs about 15$ CDN) that looks like a pair of pliers that gets the pin off in 2 seconds flat. It is well worth the price just for saving you aggravation when removing and replacing handles.