What is the use of the plus-que-parfait tense in French?

The plus-que-parfait tense is a past tense used to describe an event which happened before another event. It is a very common tense in French, but one which is often more implied or taken for granted in English. This tense enables the creation of a timeline of events and helps to establish action and consequence. 

This tense is also used to report speech, and to talk about events further in the past.

Here are some examples:

Elle mettait la robe qu'elle avait acheté la veille. She put on the dress which she had bought the day before.

Il n'a pas fait les devoirs parce qu'il avait été malade. He didn't do the homework because he had been ill. 

J'avais fait la vaisselle avant de partir. I had done the washing up before leaving. 

Conjugation

The plus-que-parfait takes the imperfect form of the auxillary verb (either avoir or étre) , and the past participle of the main verb. So, elle a acheté (past tense) becomes elle avait acheté in the plus que parfait.