Plus-que-parfait
The Pluperfect (Plus-que-parfait) indicates that an action had taken place and had been completed before another past action took place. The plus‐que‐parfait is the compound form of the imperfect and is formed by using the imperfect of the appropriate helping verb ( avoir or être) + the past participle of the verb.

Plus-que-parfait
Plus que parfait is the past “before” the past The French Post

Verb Tenses
Elements of Grammar