Heavy Fighter is a plane class that is between the fighter and the attack aircraft classes. It has a maneuverability that is lower than the fighter and higher than the attack aircraft. Heavy fighters are designed to carry more powerful weapons (such as cannons, bombs, or missles) and operate at longer ranges (mostly due to the popular twin engine design on most heavy fighters).
Heavy fighters could also serve the purpose as a ground attack aircraft because of their more powerful weapons. Some if not most also had a rear gunner to protect them from lighter, more manuverable, fighters.