A tracker mortgage, ‘tracks’ the Bank of England base rate, which currently stands at 0.1%.
For example, you might pay the base rate plus 3%
Base rate 0.1% + 3% = 3.1%
In the current mortgage market, you'd typically take out a tracker mortgage with an introductory deal period, usually of 2 years. After that period, you would be moved on to your lender's standard variable rate.
However, there are a small number of 'lifetime' trackers where your mortgage rate will track the Bank of England base rate for the entire mortgage term.