The phrase "to empty the glands" means to release or get rid of built-up or excessive fluids or substances that are stored in certain organs in the body. Full definition
Your vet or groomer may also manually empty the glands on a regular basis for you if your pup is prone to impactions. (dogcare.dailypuppy.com)
But some dogs seem unable to fully empty their glands on their own causing the glands to become impacted and uncomfortable; and as most of us can identify, dogs with impacted anal glands will usually drag or «scoot» their rear - ends along the ground (or more commonly your brand new cream - coloured carpet) attempting to empty them (pictured). (marcthevet.com)
To treat canine anal glands your veterinarian will try and express or empty the glands with his fingers. (dog-health-handbook.com)