What does it mean when a dog rubs its butt across the carpet?

A dog will commonly itch its rear end due to a full or possibly infected anal gland. (This gland is used to help dogs mark territory.) Many dogs regularly have their anal glands "expressed" (emptied) at the vet or groomer; a messy job, but it will bring your dog relief.

Less likely, the irritation may be caused by an infected gland, Allergies, or parasites. Make an appointment with your vet to diagnose and treat these problems.