Every dog is born with two anal gland sacs located on either side of their anus. These sacs fill up with fluid and produce a very strong scent which allows other dogs to identify them. These sacs are naturally expressed when your dog has a bowel movement however, some dogs are not able to express them on their own. 

When this happens, medical intervention is needed to express their anal glands. Signs that your dog may need their anal glands emptied is when he/she starts bum licking or scooting. If the anal glands are not emptied, they can become impacted and become infected.