It's literal. A cheese grater.
The Cheese Grater Image, also known as the Cheese Grater Raccoon, is a reference to a picture of Fender, an anthropomorphic ferret-fox, who has an unknown partner using a cheese grater as an instrument of sexual gratification on him. It is often regarded and referenced within the furry community as "nightmare fuel".
It involves anal penetration and blood. So yeah, don't want that. Either in real life or as a hentai tag.
(sorry for late reply, I don't use this account much)