With a repayment mortgage you pay off both the interest and capital with each monthly payment. This means at the end of the term your mortgage will be repaid providing all payments have been made.
With an interest only mortgage you only pay the interest each month and the amount of capital owed will not reduce.
This means you need to have suitable plans in place to pay off the mortgage at the end of the term. You could use sale of the mortgaged property, use savings/investments such as cash ISAs or, stocks and shares ISAs, an endowment policy or pension.