In Elden Ring, a Rune Arc is used to activate the effect of a restored Great Rune.
After defeating certain demigod bosses, the player can restore their Great Rune at a corresponding Divine Tower. Equipping that Great Rune at a Site of Grace is not enough to grant its bonus: consuming a Rune Arc activates the equipped effect.
The activation lasts until the player dies. Another Rune Arc is required to reactivate the Great Rune afterward. This makes Rune Arcs valuable consumable items, especially when a player relies on a specific Great Rune for difficult encounters.
Rune Arcs are not the same as Great Runes. A Great Rune is a major reward connected to a demigod, while a Rune Arc is the consumable used to power the equipped reward. The game also allows Rune Arcs to be used without an active Great Rune, but they then provide only a smaller general benefit.