For example if a borrower had a thirty-year mortgage loan and the first ten years were interest only, at the end of the first ten years, the principal balance would be amortized (payments for both interest and principle) for the remaining period of twenty years. The interest-only early payments are of course substantially lower than the later (interest + principal) payments. Note, however, that the later payments are substantially higher than they would be had the loan been fully amortized from the start.
But other experts warn that there is no free lunch. If you can't afford a conventional loan for the purchase, you're taking a huge, maybe ruinous, risk if you enter into a interest-only loan. All that happens with interest-only loans is that you put off the inevitable. When the principal payments are added to the mix — you're mortgage payment will spike.