The pluperfect tense (from Latin plus quam perfectum more than perfect) is a perfective tense that exists in most Indo-European languages, used to refer to an event that has completed before another past action.
In the sentence "The blind man, who knew that he had risen, motioned him to sit down again" (from Charles Dickens, Barnaby Rudge: A Tale of the Riots of 'Eighty), "he had risen" is an example of the pluperfect tense. It refers to an event (someone rises from his seat), which takes place before another event (the blind man notices the fact that the other has risen). Since that second event (the blind man's taking notice) is itself a past event and the past tense is used to refer to it ("the blind man knew"), the pluperfect is needed to make it clear that the first event (someone rises) has taken place even earlier in the past.

Examples from various languages

future tense
grammatical aspect
past tense
present tense
preterite (simple past)
Present perfect tense