A rate "lock" or "commitment" is a lender's promise to lock in a certain interest rate and a specific number of points for you for a certain period of time while your application is processed. This keeps you from working through your whole application process and learning at the end that your interest rate has gone up.
While there might be a choice of rate lock periods (from 15 to 60 days), the longer spans are generally more expensive. A lender can 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 higher than that of a rate lock of a shorter period.
In addition to opting for a shorter lock period, there are several ways you can attain the best rate. A larger down payment will get you a reduced interest rate, because you'll have a good amount of equity from the beginning. You can pay points to reduce your interest rate over the loan term, meaning you pay more up front. For many people, this is a good option..
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