A Fixed Rate mortgage is a mortgage in which your monthly payments during the initial term are 'fixed' and will remain the same.
Fixed mortgage rates are for an initial period, which can commonly be anything from 2 to 10 years. After the fixed rate period expires, your mortgage will go onto a variable rate, where your monthly payments can fluctuate. This is usually a tracker rate or your mortgage lender's Standard Variable Rate, which will not give you the same guarantee of knowing what your payments will be each month.
During the fixed rate period, your mortgage repayments will remain exactly the same, regardless of what variable mortgage interest rates do during that time.