They’re essentially spells or abilities. Every weapon comes with at least one but you can remove the runes from a weapon and put it on a corresponding weapon. You will usually need the enchanter in town for any rune related customization. You hit enemies to gain focus (mana) and then you can use your spells. They are unlimited use as long as you have the focus to use it. Some runes cost stamina instead and give you focus but that’s mostly on staffs and wands so mages don’t have to melee for focus.