When u hoop a fabric & then take the hoop off & you see the mark of the hoop still on your fabric that is not easily removed. this is called hoop burn. Usually happens on thick fabrics but can occurr on knits also.

Hoop burn is when your hoop leaves marks on the fiber that are hard to remove and in some fabrics will not come off at all. Good example will be leather or suede, that is why with some fibers they recomend that you hoop the stabilizer by itself and then place the fabric on top.