The futur antérieur (future perfect) describes an action that will already be completed before another future action or point in time — the French equivalent of "will have done". It's formed with the futur simple of avoir or être plus the past participle: j'aurai fini (I will have finished), elle sera partie (she will have left).
It appears most often after quand, dès que, or aussitôt que when the main clause is also in the future — a pattern that differs from English, which often uses a present tense there instead: quand j'aurai fini, je t'appellerai (when I've finished, I'll call you) — French requires the future perfect (aurai fini) where English uses a simple present ("I've finished").