A fixed-rate mortgage is a conventional mortgage loan that has an interest rate and monthly payment that remain the same for the life of the mortgage loan (typically 15 or 30 years total).
The interest rate is usually just a little more than the rate of the 30-year Treasury Bond at the time the mortgage is issued.
The monthly payment is a mix of principal and interest payments, with a higher amount of the payment going toward the interest in the first few years of the loan (the bank’s gotta make its money first…).
A fixed-rate mortgage is generally the safest choice for a homebuyer, because the interest rate is guaranteed not to change. When someone refinances their loan, they can change the terms of the loan, including a switch to a fixed-rate loan.