When you are offered a "rate lock" from a lender, it means that you are guaranteed to keep a set interest rate for a determined period for your application process. This saves you from going through your entire application process and discovering at the end that your interest rate has gone up.
While there are several lengths of rate lock periods (from 15 to 60 days), the longer spans are generally more expensive. You can get a longer period for your lock, but in doing so, will probably have a higher interest rate than you would with a shorter rate lock period
There are other ways to get a lower rate, in addition to choosing a shorter rate lock period. A larger down payment will result in a lower interest rate, since you'll have a good deal of equity at the start. You might opt to pay points to lower your interest rate over the life of the loan, meaning you pay more initially. To many people, this makes financial sense..
Do you have a question regarding a mortgage program?