THE PLUPERFECT (le plus-que-parfait)
The pluperfect (also known as past perfect) is a compound tense which is used to express that an action was completed in the past before another action that also happened in the past.
It is formed by using the auxiliaries être or avoir in their imperfect forms, followed by the past participle.
There are many compound tenses in French (perfect, pluperfect, future perfect, past conditional, etc.) and they all follow the same pattern (i.e auxiliary + past participle). In order to differentiate between them you need to look carefully at the auxiliary, since the past participle will be the same whatever the tense used.