In grammar, pluperfect is the verb tense that you use to talk about something that was finished or completed in the past. "They had eaten at five" uses the pluperfect.
Use pluperfect as a noun or an adjective to describe the tense you use to talk about something that didn't just occur in the past, but before the time that you're considering. This is also sometimes called the past perfect. The word pluperfect comes from the Latin phrase plus quam perfectum, "more than perfect." The Latin perfect tense refers to the past, while the pluperfect references "more than past."