What is the pluperfect tense and how is it formed?

The pluperfect tense is used when talking about events and actions that happened back in the past before a later time in the past where something else happened. It is the english equivalent of e.g. I had taken, he had arrived,...It is formed by the imperfect tense of avoir and etre + the past participle of the main verb.For example, j'avais mangé avant de partir.