An interest only mortgage is where your monthly payments only cover the interest charged on the amount you borrowed. This means that you'll need a plan in place (e.g. endowment policy, ISA, sale of property) to pay back the balance at the end of your term.
It's important that you regularly check your mortgage repayment plan to make sure it's on track to repay the loan amount when your mortgage comes to an end.
If the full amount is not paid by the agreed date your home may need to be sold to repay the mortgage.