A rate "lock" or "commitment" is a lender's promise to lock in a particular interest rate and a certain number of points for you for a specified period of time while your application is processed. This ensures that your interest rate cannot grow during the application process.
While there are several lengths of rate lock periods (from 15 to 60 days), the extended 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 period
In addition to opting for the shorter rate lock period, there are more ways you can attain the best rate. A bigger down payment will get you a better interest rate, since you will have a good amount of equity at the start. You can pay points to bring down your interest rate over the term of the loan, meaning you pay more initially. For a lot of people, this is a good option..