These are debt obligations backed by a pool of mortgages. They usually have a pass through feature. Ginnie Mae's are the most popular type of this security.
Investors have an 'undivided' interest in the pool.
The investor doesn't own any particular mortgage. Rather, he has a proportionate interest in the cash flow generated by the entire pool. Payments are received monthly by the investor. Freddie Mac's are 'Participation Certificates or PC's. Freddie Mac's are not guaranteed by the full faith and credit of the U.S. Government.