The plus-que-parfait is the past perfect tense of the indicative mood. It expresses an action that occurred before another action in the past: it could be called ''the past of the past."
The English past perfect is formed with ''had'' + past participle, while in French it's avoir or être in the imperfect + past participle.
Il était allé à Paris quand elle est arrivée. - He had gone to Paris when she arrived.
Nous avions visité le Louvre avant l'installation de la pyramide. - We had visited the Louvre before the installation of the pyramid.