In an interest-only loan, the initial period—usually ranging from 5 to 10 years—requires you to make payments that solely cover the interest accrued on the loan amount. Because you're not paying down the principal, your monthly payments will be relatively low compared to a standard amortising loan, where both principal and interest are paid.
Once the interest-only period ends, the loan "resets," and you will start making higher monthly payments that cover both the principal and the interest. This is known as the amortisation period of the loan. Since you haven't been paying down the principal during the interest-only term, these payments can be significantly higher because you'll have less time to pay off the same loan amount.