A tracker mortgage is a type of variable rate mortgage which tracks a nominated interest rate, usually the Bank of England base rate.
The actual mortgage rate you pay will be a set interest rate above or below the rate tracked. When the rate tracked goes up, your mortgage rate will go up by the same amount. And it will come down when the rate tracked comes down.
As a mortgage is secured against your home, it could be repossessed if you do not keep up the mortgage repayments.