See: Mortgage.
A note operates like a bond. It pays a stream of income, with a set interest rate and payment schedule, complete with a maturity date.
For these particular notes, the stream of income is provided by a mortgage. Each note represents a particular mortgage and can be bought and sold on a secondary market. It basically encapsulates the right to receive mortgage payments from a specific borrower paying off a specific property.
Mortgage notes can be packaged together into "mortgage-backed securities," financial instruments that represent bundles of similar mortgages. The mortgage-backed securities have less default risk than individual mortgage notes, because you're dealing with multiple mortgaes packaged together rather than a single one.
That said, mortgage-backed securities suffered a significant reputational hit during the 2007-2008 financial crisis. You know, because they nearly destroyed the U.S. economy. NBD.