The comparison to English is the past perfect tense. This is the tense that we use when we’re already talking about the past, and we want to describe an event that occured before the time we’re talking about. It’s always formed with "had + participle," although "had" contracts to ‘d in most contexts in spoken English.

I told her that I’d done it.

Je lui ai dit que je l’avais fait.

Or with être verbs:

When you got there, she had already left.

Quand tu y es arrivé, elle était déjà partie.

In general you won’t be misunderstood if you just always translate past perfect with plus-que-parfait and vice versa.

There are some subtle differences between English and French, though. I am not a native French speaker, so you should take these three points with a grain of salt.

1. Just as French makes no distinction between "present" and "present progressive," it makes no distinction between past perfect and past perfect progressive:

J’avais vu des fantômes.

could translate to either

I had seen ghosts.

or

I had been seeing ghosts.

2. Just as "depuis" takes the present in French where English uses the present perfect, "depuis" takes the imperfect in French where English uses the past perfect:

J’habite à Paris depuis deux ans.

I have lived in Paris for two years.

J’habitais à Paris depuis deux ans.

I’d been living in Paris for two years.

3. In English, the past perfect is often not used in stand-alone sentences, even when logically it is correct, if the context is clear. In French, the plus-que-parfait is used a little more often in these contexts:

Why did you do that?

Because I noticed that you needed it.

In English, "I’d noticed" would sound stilted here, although not exactly wrong. But in French, it’s more natural:

Pour quoi tu as fait ça ?

Parce que j’avais remarqué que tu en avais besoin.