http://www.yourmortgage.co.uk/mortgage_basics/tracker_mortgages
Tracker mortgages
This type of product follows the Bank of England Base Rate or the lender's Standard variable rate (SVR's), plus or minus a certain percentage. For example, if your two-year tracker has a rate of Base Rate (BBR) plus 0.5%, if the Base Rate is 1.5%, your product rate will be 2.0%.
However, if the Base Rate drops to 1.0%, your rate will also fall to 1.5%. If your mortgage tracks your lender's SVR it may not necessarily change in line with the Base Rate, however a Base Rate tracker is guaranteed to. It is also possible to get mortgages that track other rates, such as LIBOR (London Interbank Offered Rate; the rate at which banks lend to each other), but these products are nowhere near as common.