Barrows is a minigame that allows a player to gain 6 different sets: 4 melee, 1 range, and 1 mage set. There are 6 brothers you have to kill in this minigame: Ahrim the Blighted, Guthan the Infested, Karil the Tainted, Torag the Corrupted, Verac the Defiled, and Dharok the Wretched.
To get to barrows go to the minigames teleport in your magebook and click "Barrows". You will need a spade, which you can get from the general store for normal players or Adam for players using the Iron Man game mode, to dig into the brothers burial chambers located on each of the 6 hills in the minigame.