When you are promised a "rate lock" from a lender, it means that you are guaranteed to get a particular interest rate over a determined period while you work on your application process. This protects you from getting through your entire application process and learning at the end that your interest rate has gotten higher.
While there might be a choice of rate lock periods (from 15 to 60 days), the extended ones are usually more expensive. You can get a longer period for your lock, but in choosing this option, will likely have a higher interest rate than you would with a shorter period
In addition to opting for a shorter lock period, there are more ways you can attain the lowest rate. A bigger down payment will get you a reduced interest rate, because you'll be starting out with a good deal of equity. You can pay points to lower your rate for the life of the loan, meaning you pay more initially. For a lot of people, this makes sense and is a good deal..