With any mortgage you take out, you have to pay a lender interest at an agreed rate for borrowing the money, as that’s the business they are in! With an interest only mortgage you only pay the interest charge every month. You don’t have to make monthly repayment towards the money borrowed, which means the monthly payments are the cheapest they can be.
However, the lender will only allow the mortgage to continue for a set amount of time known as a mortgage ‘term’. Come the end of the term, your will need to have found another way to have paid the mortgage back. This could be selling the property you are in, another property or selling other assets. Of course this means that this should only be considered if you are comfortable with doing something like this.