When you're telling a story, you describe the setting at a certain time in the past. If you then want to describe something that happened before the setting of the story, you use "I had ___". Here's an example:
I was almost at the train station when I realized that I had left my wallet in my other purse.
You can also talk about things that didn't happen before the setting of the story. In this sentence:
Believe it or not, I had never even traveled abroad when I moved here.
...the basic setting of the story is "when I moved here". The speaker didn't have any travel experience then, although she might have some now.