A "fixed rate" is an interest rate that is fixed for a specified period of time.
Having a loan based on a fixed rate of interest means that you know what your monthly repayments will be during the fixed interest period - they won't change even if interest rates generally go up or down. After the end of the fixed interest period, the lender's then standard variable rate of interest will usually apply to the loan for rest of the term and the monthly repayments can be higher or lower than before the end of the fixed interest period.