When you're offered a "rate lock" from your lender, it means that you are guaranteed to get a set interest rate for a certain number of days while you work on the application process. This means your interest rate can't grow as you are going through the application process.
While there might be a choice of rate lock periods (from 15 to 60 days), the extended ones are typically more expensive. The lending institution may agree to freeze an interest rate and points for a longer period, such as sixty days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of fewer days.
In addition to going with a shorter rate lock period, there are other ways you may be able to get the lowest rate. A larger down payment will give you a lower interest rate, because you'll have a good amount of equity at the start. You could opt to pay points to reduce your rate over the life of the loan, meaning you pay more up front. One strategy that is a good option for some is to pay points to improve the rate over the life of the loan. You'll pay more up front, but you will save money in the end.
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