An Interest-Only mortgage requires that you only repay the interest during the term of the mortgage. So you won’t actually be paying down the mortgage balance, which remains at the same level throughout the mortgage term.
Your monthly payments will be lower than a repayment mortgage – and this can be advantageous for some people. You will however need to ensure that you have a repayment plan in place as you’ll need to repay the actual balance all at once at the end of the mortgage term.