In grammar, the past perfect is a verb tense you use to talk about something that happened and was finished in the past. The sentence "He had arrived the day before" is in the past perfect tense.
The past perfect — which you can also call the "pluperfect" — always includes the word had. While a sentence like "We saw that movie" is in the simple past tense, "We had already seen that movie" is in the past perfect. The term past perfect reflects the fact that this peculiar verb tense combines the past with what's known as the perfect in grammar, demonstrating that an action happened earlier than the time you're talking about.