Hi there -
They are just ways of sending a note to someone without posting it in the forum or having to send a private message. Think of it as posting on their wall, as in Facebook, or adding a comment to someone's MySpace page.
They are public, so anyone can see them, so keep that in mind when sending.
Emily
Hello Okjun- I believe notes are a way to say hi, express our thoughts with icons. You would get them from friends. I did not start getting any until I had friends. I hope this helped some, I will research it, let you know if there is more to it.