cash back to the user is a cut of the affiliate revenue/commission they get for the sale. this is commonly how all third party ecommerce cash back programs work (honey, klarna, etc.)

the video explains how honey has the same thing but the cut they give you seems to be tiny - like less than a dollar to users for a $30 commission for them as one example.

i dont know if this is common practice or not but i'm not a user of these extensions personally.