With an interest only mortgage you will only make payments towards the interest on the amount you've borrowed. This means that you will still owe the full amount borrowed at the end of your mortgage term.
To qualify for an interest-only mortgage you’ll need to demonstrate to your lender that you have a repayment plan in place and will be able to pay off the loan amount once the term is over.
Lending criteria for interest-only mortgages are usually much tighter than for a repayment mortgage. Your lender will probably do extensive affordability checks, and may require a bigger deposit.