The Past Perfect Tense or Pluperfect corresponds to the English had + past participle and refers to something that had already happened when something else happened. It consists of the imperfect of haben or sein and a past participle and is comparable to the present perfect tense.
Present perfect: Ich habe in Wiesbaden gewohnt. I (have) lived in Wiesbaden.
Past perfect: Ich hatte in Wiesbaden gewohnt. I had lived in Wiesbaden.
Present perfect: Was ist passiert? What (has) happened?
Past perfect: Was war passiert? What had happened?