It’s an ability called Divine Intervention that allows you to call upon your God to choose from: have an instant long rest (resurrecting all fallen companions in the process), get a legendary weapon, a chest full of potions and supplies, or deal a huge AOE of radiant damage. You get it at level 10, max level is 12.
It’s one time use because in D&D it can only be used again after 7 days if it works, while it works perfectly every time in bg3.