A 30-year fixed rate mortgage is a loan that's outstanding for 30 years, where the rate is fixed for the whole time. 30-year fixed rate mortgages amortize over time, which means that a portion of the principal is paid back every month. The monthly payment is the same every month for 30 years, and the end of the 30 years, the loan is paid off in full. These types of mortgages are the most popular in the United States.