A rate "lock" or "commitment" is a promise from the lender to set a particular interest rate and a certain number of points for you for a certain period of time during your application process. This means your interest rate won't get higher during the application process.
While there may be a choice of rate lock periods (from 15 to 60 days), the longer ones are generally more expensive. A lender can agree to lock in an interest rate and points for a longer span of time, like sixty days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of a shorter period.
In addition to opting for the shorter lock period, there are more ways you are able to get the lowest rate. A larger down payment will get you a better interest rate, since you will have a good deal of equity at the start. You can pay points to reduce your rate over the loan term, meaning you pay more initially. To many people, this makes financial sense..
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