Berserk is a Warrior only command which is used to attempt to enter a state of mind where the warrior will deal damage faster and pay no regard to damage done to them.
While berserking, a warrior will get stat boosts of +1 to strength and dexterity. These do not stack with other stat boosts such as those received from iron gauntlets.
If you wait for the effect to wear off normally, you will suffer a stat penalty of -2 to strength and constitution, and will not be able to berserk again for a time. The Restore ritual will remove the stat penalty, but you'll still have to wait to berserk again.