Both GSEs buy home loans from banks and credit unions on the secondary mortgage market. They keep some mortgages on their books and make money from the interest. But usually, Freddie and Fannie pool many mortgages together into bundles called mortgage-backed securities and then sell them to investors. Both guarantee that investors will get paid even if some homeowners in the pool default, making mortgage-backed securities extremely safe investments.
By buying loans from financial institutions, Freddie and Fannie improve liquidity in the mortgage market. In other words, lenders don't have to keep loans on their books for the entire life of the mortgage. Instead, banks and credit unions can resell mortgages to Freddie and Fannie, freeing up cash for them to make additional loans.
Freddie and Fannie will only purchase loans that meet conventional standards, which they're responsible for setting. The standards for a conventional loan are virtually identical. They include a 620 minimum credit score, a debt-to-income ratio of 45% or less, and a 3% down payment.