Interest rates can often fluctuate without advance notice, depending on the market. Locking in a rate protects you from the time that your lock is confirmed to the day that your lock period expires. A lock is an agreement by the borrower and the lender and specifies the number of days for which a loan's interest rate and points are guaranteed. Should interest rates rise during that period, your lender is obligated to honor the committed rate. Should interest rates fall during that period, the borrower must honor the lock.