Dogs with double coats boast and undercoat and an outer coat. In short, their coats are made up of long, guard hairs under which there is a denser, woollier and usually much softer undercoat. The denser the undercoat happens to be, the fluffier a dog's coat tends to be and the more grooming they need to prevent tangles and mats from forming.
The outer guard hairs serve to repel any moisture and they help get rid of any dirt whereas the softer undercoat acts more as an insulation that keeps dogs warm during the colder winter months and cooler when the weather is hot in the summertime. As such, a double coat should never be shaved because it takes away the barrier that protects dogs from the damaging UV rays of the sun.