A rate "lock" or "commitment" is a promise from the lender to hold a particular interest rate and a particular number of points for you for a certain period while your application is processed. This ensures that your interest rate can't go up during the application process.
Although there are several lengths of rate lock periods (from 15 to 60 days), the extended ones are usually more expensive. A lending institution will agree to freeze an interest rate and points for a longer period, say sixty days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of a shorter period.
In addition to opting for a shorter lock period, there are other ways you are able to attain the lowest rate. The bigger down payment you can make, the lower the rate will be, as you will be entering the loan with more equity. You can pay points to improve your interest rate over the life of the loan, meaning you pay more initially. One strategy that makes financial sense for some is to pay points to bring the rate down over the life of the loan. You will pay more up front, but you'll come out ahead, especially if you keep the loan for the full term.
Do you have a question regarding a mortgage program?