With a capital repayment mortgage, your monthly payments will pay off part of the loan each month as well as paying the interest, which means the mortgage will be fully repaid by the end of the term if you keep up your repayments.
With an interest-only mortgage, your monthly payments only cover the interest on your loan, which means the loan itself isn’t repaid over time. You will need to find another way to repay the capital (the loan amount itself) in full by the end of the loan term.