You usually find it while stroking your dog: a hard patch behind the ear, a firm clump under the front leg. The first impulse is to fetch the brush or run a bath. The second is exactly what you shouldn't do.
Water makes mats worse. A matted dog should never be bathed before the coat is combed out or the mats are removed. Wet mats tighten and get firmer, and the skin beneath dries badly.
The brush has limits. Small, loose knots can often be worked out patiently. Hold the hair between the knot and the skin so it doesn't pull. A firm mat that reaches the skin usually can't be brushed out without pain.
Scissors are more dangerous than they look. Mats often sit tight against the skin, which is hard to see underneath. A cut happens quickly. So dense mats shouldn't be removed with scissors at home.
Mats shouldn't wait. They can pull, irritate the skin and hide what's going on beneath. The ASPCA names matted coats as a possible trigger for hot spots. If a spot smells, is damp or hurts, a vet should look at it.
What happens at the salon. First we check how dense the mats are and how the skin is doing. What can be loosened gently gets loosened. If the coat is too dense, a short cut is sometimes the kinder option. Hair grows back, and the dog is free of the pulling.
So it doesn't happen again. Mats form mainly where there's friction: behind the ears, under the collar or harness, in the armpits, on the back legs. Comb these spots more often, right down to the skin.