Mortgage interest is the cost you pay to borrow money from a lender to buy a home. When you take out a mortgage, the lender charges you interest on the amount you borrow. This interest is usually expressed as a percentage of the loan and is added to your monthly payments. The interest rate can vary based on factors like your credit score and the type of mortgage you choose. Over time, as you make payments, a portion goes toward paying off the principal (the original loan amount) and another portion covers the interest.