In an interest-only mortgage, your monthly payments cover the cost of the interest, but you do not pay off the capital value of your home. This is a good way of keeping repayment costs down, but may be a gamble in the long run.
At the end of the mortgage term you will be required to pay off any outstanding amount. Usually this will be the amount you borrowed in the first place. If you cannot pay off the remainder, you will be at risk of losing your home.
You may choose to save over the term of the mortgage. This does not have to be arranged through your lender, although they may offer you a number of investment options designed to clear your debt.
