Mortgage rates are constantly fluctuating, so once underwriting begins on your loan, it’s important to lock in a good rate in case they go up before your closing. A mortgage rate lock is the interest rate at which your lender offers to secure your loan. Once you have locked in your rate, it won’t change—which means it can be a gamble if rates drop again. Note that your lender will only offer to lock in your rate once you have been fully approved for your loan, not just preapproved, and they can back out if any issues arise with your closing.