When you are promised a "rate lock" from your lender, it means that you are guaranteed to get a certain interest rate for a certain number of days for your application process. This saves you from working through your entire application process and learning at the end that your interest rate has risen higher.
Although there are various lengths of rate lock periods (from 15 to 60 days), the longer spans are generally more expensive. The lending institution will agree to hold an interest rate and points for a longer period, say 60 days, but in exchange, the rate (and sometimes points) will be higher than that of a rate lock of fewer days.
There are other ways to get a good rate, besides going with a shorter rate lock period. A larger down payment will result in a reduced interest rate, since you'll have a good amount of equity at the start. You may choose to pay points to lower your interest rate over the term of the loan, meaning you pay more initially. For a lot of people, this is a good option..
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