A Lifetime Tracker mortgage is one that tracks the Bank of England base rate for a certain percentage for the lifetime of the mortgage loan. 
A Lifetime tracker deal may be beneficial at times of low interest rates but at times of high interest rates you may end up with a large monthly payment for your mortgage.
Lifetime tracker deals usually offer some level of flexibility with common features including an overpayment facility with the ability to make overpayments on your mortgage without incurring additional penalties.