the Quote (Spoilers)

[Nutasarëiel]

This excerpt has been released to the public:

Dumbledore lowered his hands and surveyed Harry through his half-moon glasses.
"It is time," he said, "for me to tell you what I should have told you five years ago, Harry. Please sit down. I am going to tell you everything."

I have an idea about this. What is the one thing that still remains a major mystery in the plot of the story? It's, Why did Voldemort want to kill Harry and his father to begin with (and why was his mother not meant to die)? I have a feeling that this is what the excerpt is about. I remember at one point in the first book (I don't remember the context) Harry asked Dumbledore about this, and he responded that he would tell him when he got older. I'm not even going to try to make my own explanation of why Voldemort did what he did, but it's a start. What do you think? Any other theories?