What is it called when you have to pay off the balance before the bank starts charging interest?

This is typically referred to as a "grace period." During this time, you can pay off your balance without incurring interest charges. Grace periods are commonly associated with credit cards, where the period usually lasts from the end of a billing cycle until the payment due date, provided the previous balance was paid in full. If the balance is not paid off within this period, interest is then charged on the remaining amount.