When you're offered a "rate lock" from your lender, it means that you are guaranteed to get a particular interest rate for a certain number of days while you work on the application process. This saves you from working through your entire application process and finding out at the end that the interest rate has gone up.
While there might be a choice of rate lock periods (from 15 to 60 days), the extended spans are usually more expensive. You can get a longer period for your lock, but in doing so, will probably have a higher rate than you would have with a shorter period
There are other ways to get a low rate, besides opting for a shorter rate lock period. The bigger down payment you pay, the better your interest rate will be, since you will have more equity from the start. You might opt to pay points to reduce your interest rate for the loan term, meaning you pay more initially. For many people, this makes financial sense..
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