The Putnam Mortgage Securities Fund Class B (PGSBX) is a mutual fund that primarily invests in mortgage-backed securities. These are debt securities that are backed by a pool of mortgages, providing investors with a steady stream of income. The fund is managed by Putnam Investments and is designed for investors seeking a balance of income and potential for capital appreciation. As a Class B fund, it may have higher fees and expenses compared to other classes of the same fund.