The Rage Meter is loosely based on Calamity Mod's Rage Meter. The main sources of Rage are from Dealing and Taking damage. By default, you gain 15% of damage you deal as rage, this value can be modified in the config. You also gain 100% of Damage you take as Rage. Both of these values can be augmented with curios.
By default, you can store 100 Rage. Every 100 Rage Points provide an additional level of the Enraged Effect. You may click on the link for more info as to what the effect actually does.
Also by default, Rage is automatically proc'd whenever you fill up the Rage Meter. This can be circumvented by equipping the Anger Management curio.
Here you can see how the Rage Meter would look like in game.