When you are offered a "rate lock" from the lender, it means that you are guaranteed to keep a specific interest rate for a determined period for the application process. This saves you from working through your whole application process and learning at the end that the interest rate has risen higher.
While there might be a choice of rate lock periods (from 15 to 60 days), the longer ones are generally more expensive. The lender can agree to lock in 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 fewer days.
There are more ways to get a good rate, besides opting for a shorter rate lock period. The bigger the down payment, the lower the interest rate will be, because you will have more equity from the start. You might opt to pay points to improve your rate for the life of the loan, meaning you pay more initially. For many people, this is a good option..
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