A rate lock is the lender's promise that the mortgage's interest won't surpass a certain rate if the loan is closed by a deadline. For example, if you lock the rate at 5 percent 21 days before closing and rates rise over the next three weeks, your loan's rate will not change if you close on time. If you decided not to lock and the rates increased, you would pay the higher rate.

Most lenders do not charge a fee to lock a rate within 30 days of the scheduled closing. Fees are common when the lock is beyond 30 days, and especially when it is beyond 60 days. The fees vary, and some lenders will refund all or part of the rate lock fee at closing.

Caps usually cover long locks, such as a 180-day lock. Many long-term locks have a float-down option, which allows the borrower to seize and lock a lower rate shortly before closing if rates have dropped in the meantime. The interest rate is not yours until you’ve locked your rate. Mortgage rates change every day, sometimes more than once, and until you’ve told your lender to lock the rate on your loan your interest rate will change daily, too.

If you are happy with the current rate, then you should probably lock it. That will protect you in case interest rates rise. If you want the rate to be lower, you can hold off, but your rate might go up as well as go down while you wait.

When you do lock, make sure to get a written confirmation from your lender that the rate is locked. This confirmation should contain the rate and expiration date of the lock. This will ensure there are no misunderstandings about the details of the loan at a later date.