When you are promised a "rate lock" from your lender, it means that you are guaranteed to keep a certain interest rate for a determined period for your application process. This ensures that your interest rate cannot rise as you are working through the application process.
Although there are several lengths of rate lock periods (from 15 to 60 days), the longer spans are typically more expensive. You can get a longer period for your lock, but in doing so, will most likely have a higher rate than you would have with a shorter span of time
There are more ways to get a better rate, besides choosing a shorter rate lock period. The larger the down payment, the lower your interest rate will be, because you will be entering the loan with more equity. You can pay points to bring down your interest rate over the life of the loan, meaning you pay more initially. One strategy that is a good option for some is to pay points to bring the rate down over the life of the loan. You'll pay more up front, but you will come out ahead in the long run.
Do you have a question regarding a mortgage program?