A Tracker mortgage has is a variable rate which is linked directly to the Bank of England Base Rate. Your mortgage will track the Base Rate at a margin which will remain constant for the duration of either an an initial period or for the life of the mortgage.
To illustrate the point, if Bank of England Base Rate is at 2.0%, you may have a product which is marketed at Base + 1.5% for two years. This would mean that your current rate is 3.5%. If the Bank of England subsequently increases rates to 2.5%, your mortgage rate would also increase by the same margin, making your mortgage pay rate 4.0% along with the corresponding rise in monthly payments. Similarly, if the Bank of England reduces the rates to 1.25%, your pay rate would also go down to 2.75% with your monthly payments getting cheaper as a result.