What is a note?

[Matt DuSold]

Yes I know, simple question. I just cant seem to understand what it is trying to read about it so maybe someone can break it down for me.

I was reading through the forums and saw hoe Rich used to accept notes as commission and made a pretty penny, so I was intrigued. There's just one problem, I have no clue what the heck a note is. If someone could please explain it to me as if Im in 4th grade I would appreciate it. Thanks!