Dogs have Anal glands which are scenting glands, the two small pouches are on either side of their anus. They make a smelly, oily, brown fluid that dogs use to identify each other and mark their territory. It’s why they often sniff each other’s behinds. Anal sac disease begins as an uncomfortable impaction and can progress to an infection or abscess.
If your dog is scooting its bottom along the floor, several times a day or is irritated in that region please consult your vet.
Anal glands should not be expressed externally unless recommended by a vet and expressed by the vet or trained professional.