I also recommend using a fine tooth comb to check for mats and snarls. Again, be very careful not to jab the comb into the skin or to pull on the hair heavily, as this will irritate and injure the skin. When brushing, be sure to lift the coat, section by section from the bottom, moving on to another section only after having entirely finished with the spot where you began. Always face the bristles of your brush at an angle away from the skin. Follow through with the comb to check your progress.
Some dogs have fluffy undercoat that clumps and mats as it falls out.
Huskies and Shepherds are especially prone to this. Be sure to brush your dog thoroughly all the way to the skin to remove this and prevent skin disorders and discomfort to your pet. Never bathe your pet without brushing out its coat first. Once mats get wet, they become nearly immovable. I would love to answer every possible question here, but there is not enough room to address them all.