When you're promised a "rate lock" from your lender, it means that you are guaranteed to get a set interest rate for a determined period for your application process. This protects you from going through your whole application process and finding out at the end that the interest rate has gotten higher.
Rate lock periods can vary in length, anywhere from 15 to 60 days, with the longer period usually costing more. A lending institution will agree to lock in an interest rate and points for a longer period, say sixty days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of fewer days.
There are other ways to get a lower rate, besides going with a shorter rate lock period. The more the down payment, the better the interest rate will be, because you will be entering the loan with more equity. You might choose to pay points to bring down your rate over the term of the loan, meaning you pay more initially. To many people, this makes financial sense..
Do you have a question regarding a mortgage program?