In Soulsplit, a Set is a group of items that go with each other to create a look, and in some cases provides extra bonuses for having a complete set equipped.
Barrows equipment is a good example of a set that provides extra bonuses. When all four of one of the Barrows Brother's sets is equipped by a player, a special passive ability is unlocked. In the case of Dharok's equipment, a complete set will add the chance to hit a certain number of damage. This damage increases the lower the player's health is. This is just one example of how sets can aid you in combat.
Other sets don't provide special abilities or provide few combat bonuses. Some sets are worn by players for appearance, such as gold-trimmed Wizard robes or. Some sets do provide combat bonuses, such as Gilded armor, but are too expensive to risk losing.