When you are offered a "rate lock" from the lender, it means that you are guaranteed to keep a certain interest rate over a certain number of days while you work on your application process. This means your interest rate can't grow during the application process.
Although there may be a choice of rate lock periods (from 15 to 60 days), the longer ones are typically more expensive. The lending institution can agree to hold an interest rate and points for a longer span of time, say 60 days, but in exchange, the rate (and sometimes points) will be higher than that of a rate lock of fewer days.
In addition to going with the shorter rate lock period, there are several ways you are able to score the lowest rate. A bigger down payment will give you a lower interest rate, since you'll be starting out with more equity. You could choose to pay points to lower your rate over the term of the loan, meaning you pay more up front. One strategy that makes financial sense for many people is to pay points to improve the interest rate over the life of the loan. You'll pay more initially, but you will come out ahead in the end.
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