What is a note: Wiki says "In the United States, a mortgage note (also known as a real estate lien note, borrower's note) is a promissory note secured by a specified mortgage loan; it is a written promise to repay a specified sum of money plus interest at a specified rate and length of time to fulfill the promise."
Our company purchases notes on small balance real estate (SBRE) with typical property values of $30k-$120k. These small notes are "overlooked" by the large investors because they don't have nearly the profit that a California $500k note would have.
We focus on "distressed" notes, buying them at a discount, so that in the event of a real estate downturn or other unforeseen event, our investment has a good chance to still be less than the value of the real estate that backs the note.
Here is a good, simple article on the note investment business. We can point you to books, podcasts, and even the formal training if you want more information.
https://www.biggerpockets.com/renewsblog/2013/02/19/five-advantages-of-note-investing