It is a bank account, but it decreases the interest you pay on your home loan rather than earning interest for you. Every dollar you put in your offset account will lessen the interest amount you pay while in your account.
Since you don’t earn interest on an offset account, alternatively, the savings you have in your offset decreases the amount of loan interest you pay. This is typically a much higher interest rate than you would earn in your bank account. Therefore, funds in your offset account help you save more than you would make in interest.