Lifetime mortgages involve taking out a mortgage against your property. But instead of making repayments against the amoutn borrowed, the capital remains outstanding and is repaid when you pass away. In addition, the interest accumulates ("rolls up") with the capital. Therefore, you receive an immediate cash lump sum, with no obligation to repay any amounts during your lifetime.
Interest only mortgages are similar to lifetime mortgages, but instead of the interest "rolling up", the interest is paid periodically. As a result, the amount which must be repaid from the proceeds of your home is far less than with a lifetime mortgage.