Plus-que-parfait

Passé composé =  Perfect
Imparfait     =  Imperfect
Plus    means     more
que     means     than
parfait means     perfect

Plus-que-parfait means more than perfect.

If an action takes place before the perfect, it will be described
in plus-que-parfait.
If there are two actions in the past, the earliest action will be
in plus-que-parfait and the second action if completed will be in
passé compose and if not completed in imparfait.

Formation
Conjugate avoir in imparfait + the past participle of the main verb.
Conjugate être  in imparfait + the past participle of the main verb.
All the other rules of passé composé will apply.