When you're promised a "rate lock" from a lender, it means that you are guaranteed to get a particular interest rate over a determined period while you work on your application process. This ensures that your interest rate won't rise during the application process.
Although there can be a choice of rate lock periods (from 15 to 60 days), the extended spans are generally more expensive. The lender may agree to hold an interest rate and points for a longer period, say sixty days, but in exchange, the rate (and sometimes points) will be more than with a rate lock of a shorter period.
There are more ways to get a lower rate, in addition to agreeing to a shorter rate lock period. A bigger down payment will result in a reduced interest rate, since you will be starting out with a good deal of equity. You can pay points to reduce your interest rate for the life of the loan, meaning you pay more initially. For many people, this makes financial sense..
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