Q:

What does "you're the one" mean in this context?

Daughter brought her boyfriend Mick to home, so her father & Mick
started conversation.

Mick: I know. You were in music band, right?
Father: You're impressing me already.
Mick: Well, I have your CDs.
Father: You're the one.

My Understanding: Still no one buy his CD's by saying one refers Mick is the first one to buy CD. since s word added to CD, that is why I am confused.

A:

It's possible that Mick is the one who bought the CDs and that this is somehow noteworthy in the mind of the father. However, often when someone is "the one", that person is the most suitable, the best candidate, for something. 
In particular, in a romantic context, "the one" is a true love, the person that someone is going to marry. Here, the father is impressed by Mick, so he thinks that Mick is "the one" for his daughter, the person she should marry. Keep in mind that he's joking or being sarcastic. 

