Present Perfect is used for actions which happened in the past but have some influence for the present:
I have done it and now I am happy
What is "have done" action? It's something what happened before "I am happy". I am happy but why? Because I have done it. They are connected.
If something happened in the past and doesn't have influence for present, we can say the same in Past SImple:
I did it and now I am happy
What is "did" action? It's something what happened before "I am happy". I am happy but why? It's unknown but the fact of me doing it in the past isn't connected with my happiness now. I am happy because of something else but not "I did it" makes me happy now.
It's clear, it's okay.
The trouble starts with Past Perfect:
I had done it and then I was happy.
What is "Had done" action? It's something what happened before "I was happy". But was it the real reason which I was happy for? Was it the analogy with "I have done"? Or it is an analogy with "I did" when something was earlier and doesn't have influence?
For Present we have two options:
1) Something was in the past and matters now - I have done 2) Something was in the past and doesn't matter now - I did
But we don't have the same system for Past and Future. If it were something like:
1) Something what was before something in the past and didn't matter - I had done
2) Something what was before something in the past and mattered - I had had done
It's just an exmaple of how it could be. So what does Past Perfect mean? It's like something what was before some other action or moment and doesn't have connection with it(=Past Simple) or it was earlier some other action or moment and it has this connection, has this result but only for Past (=Present Perfect)