A rate "lock" or "commitment" is a lender's promise to lock in a certain interest rate and a certain number of points for you for a certain period during your application process. This means your interest rate cannot rise during the application process.
Rate lock periods can be various lengths of time, anywhere from fifteen to sixty days, with the longer ones usually costing more. A lending institution will agree to lock in an interest rate and points for a longer period, like sixty days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of a shorter period.
There are more ways to get a lower rate, in addition to going with a shorter rate lock period. A bigger down payment will get you a reduced interest rate, because you'll be starting out with more equity. You can pay points to improve your rate over the term of the loan, meaning you pay more up front. One strategy that makes financial sense for many people is to pay points to bring the rate down over the term of the loan. You'll pay more up front, but you'll save money, especially if you don't refinance early.
Do you have a question regarding a mortgage program?