See BBG 25.24.
The pluperfect tense (from Latin meaning more than perfect) is roughly equivalent to the past perfect tense in English. In English, we translate these with the helping verb “had”. The pluperfect refers to an event that has completed before another past action. Take this sentence,

The blind man, who knew that he had risen, motioned him to sit down again.

“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). Because 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.
GGBB writes:

“As was stated in the general introduction to both the perfect and the pluperfect, for the most part, these two tenses are identical in aspect though different in time. That is, both speak of the state resulting from a previous event—the perfect speaking of existing results in the present (with reference to the speaker), the pluperfect speaking of existing results in the past (as this tense occurs only in the indicative mood). Thus, it may be said that the pluperfect combines the aspects of the aorist (for the event) and the imperfect (for the results).

To put this another way, the force of the pluperfect tense is that it describes an event that, completed in the past, has results that existed in the past as well (in relation to the time of speaking). The pluperfect makes no comment about the results existing up to the time of speaking. Such results may exist at the time of speaking, or they may not; the pluperfect contributes nothing either way. (Often, however, it can be ascertained from the context whether or not the results do indeed exist up to the time of speaking.)  …

There are only 86 simple pluperfects in the NT. In addition, there are a number of pluperfect periphrastic constructions (i.e., εἰμί in the indicative + a perfect participle).”

GGBB p. 583.