When you're offered a "rate lock" from your lender, it means that you are guaranteed to keep a certain interest rate over a certain number of days for the application process. This ensures that your interest rate will not grow during the application process.
While there are various lengths of rate lock periods (from 15 to 60 days), the longer ones are usually more expensive. A lending institution can agree to freeze an interest rate and points for a longer span of time, say 60 days, but in exchange, the rate (and sometimes points) will be more than with a rate lock of a shorter period.
In addition to choosing a shorter rate lock period, there are several ways you can score the lowest rate. A bigger down payment will get you a lower interest rate, because you will have more equity at the start. You can pay points to improve your rate over the loan term, meaning you pay more initially. One strategy that is a good option for many people is to pay points to reduce the interest rate over the life of the loan. You'll pay more up front, but you'll save money, especially if you keep the loan for the full term.
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