A fixed-rate mortgage locks in your interest rate on day one and keeps it for the entire life of the loan. Your principal-and-interest payment in year one is identical to your payment in year thirty — no surprises, no resets, no fine print about adjustment caps.
That predictability is the whole appeal. You can budget decades ahead, and if market rates rise, your payment doesn't care. If rates fall instead, you're not stuck either — you can refinance into a lower rate.
The most common flavors are the 30-year and 15-year terms. The 30-year keeps the monthly payment lower; the 15-year costs more per month but pays far less interest overall and builds equity much faster — see the trade-offs in 15 vs. 30-year.
One nuance: your total payment can still change slightly from year to year if the taxes or insurance in your escrow account rise. It's the principal and interest that are carved in stone. Compare current pricing on 30-year fixed rates.