Now we're getting to the end of the grooming process. The anal sacs, or glands, are located on each side of the anus (if your Lab's rear were a clock, you would find the anal glands at the 5:00 and 7:00 positions). The anal glands produce a fluid that's excreted when a dog defecates. Sometimes the glands become clogged. If your Lab is scooting his rear on the ground or frequently biting and licking at it, he may have intestinal parasites or impacted anal glands.
Impacted anal glands require veterinary intervention in the form of emptying, or expressing, the glands. The vet can show you how to do this at home should the situation recur. It can be a stinky job, though, so if you're squeamish you may prefer to just take the dog in whenever the glands need to be expressed.