What are Mortgage Securities?
Mortgage securities are a special class of REITs - various mortgages are bundled together to form a single investment trust (mREIT). These mortgages are securities by the underlying properties, but these REITs do not own or operate the properties. So if you invest in them, you are not owning properties, you earn interests from real properties (that's where the "debt" of the Public Debt comes from).
How to Analyze mREITs?
If you want to analyze mREITs, it could get technical very quickly because an agency mREIT buys and holds agency residential mortgage-backed securities (MBS), financed through repo, and hedged with a variety of derivative or cash positions. Don't think there is no risk because these assets are already backed up by MBS, there is huge interest rate risk because borrowers of the mortgages could prepay if interest rate drops!
For a better analysis, I recommend this seekingalpha article - How To Analyze The Value In Agency Mortgage REITs.