When you're promised a "rate lock" from your lender, it means that you are guaranteed to keep a specific interest rate for a determined period while you work on the application process. This protects you from working through your entire application process and finding out at the end that the interest rate has gotten higher.
Although there are several lengths of rate lock periods (from 15 to 60 days), the extended ones are typically more expensive. A lending institution will agree to hold an interest rate and points for a longer span of time, such as sixty days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of fewer days.
There are other ways to get a lower rate, in addition to choosing a shorter rate lock period. A bigger down payment will give you a better interest rate, because you will have more equity from the beginning. You might opt to pay points to lower your interest rate for the loan term, meaning you pay more up front. One strategy that makes financial sense for some is to pay points to bring the rate down over the life of the loan. You are paying more initially, but you will save money, especially if you don't refinance early.
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