Mortgage interest rates can change daily, sometimes hourly. A lock-in or rate lock on a mortgage loan means that your interest rate won’t change between the offer and closing, as long as you close within the specified time frame and there are no changes to your application. If rates go down, you may have the option to withdraw from the agreement. The probability of such a withdrawal is known as a fallout risk for the lender. You should take great care, however, to ensure that the lock agreement allows for withdrawal.