When you are offered a "rate lock" from the lender, it means that you are guaranteed to get a particular interest rate for a determined period for your application process. This keeps you from getting through your entire application process and learning at the end that the interest rate has gone up.
While there may be a choice of rate lock periods (from 15 to 60 days), the extended spans are typically more expensive. You can get a longer period for your lock, but in choosing this option, will likely have a higher rate than you would with a shorter span of time
In addition to opting for a shorter lock period, there are other ways you are able to attain the best rate. A larger down payment will result in a better interest rate, because you will be starting out with a good deal of equity. You may opt to pay points to lower your rate for the loan term, meaning you pay more up front. One strategy that is a good option for some is to pay points to bring the rate down over the term of the loan. You'll pay more initially, but you will save money in the end.
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