The pluperfect is a compound tense consisting of the imperfect tense of either of the auxiliary verbs avoir and être followed by the past participle of the relevant verb.
e.g. Elles étaient sorties quand il est passé, which means , They were out when he passed by.
The pluperfect is used for past action and describes an action which precedes another in the past. It, therefore, expresses the idea ‘had done’ in English. In fact, usage of the pluperfect is identical in both languages.
e.g. J’avais déjà pris ma place lorsque le professeur est entré, which means , I had already taken my seat when the teacher entered.