With a fixed rate mortgage, the rate will be fixed for the initial period. This period is typically between 1 and 5 years, although some lenders may offer longer. You are likely to incur an early repayment charge if your mortgage is paid off within this time. Once this initial period comes to an end you are likely to go onto the lenders standard variable rate.
Choosing a fixed product has the advantage that you know exactly how much your mortgage will be costing you for that initial period. If interest rates were to go up your payment will not increase, however if interest rates were to go down you would not benefit from this.