A rate "lock" or "commitment" is a promise from the lender to hold a specific interest rate and a certain number of points for you for a specified period of time while your application is processed. This saves you from working through your whole application process and discovering at the end that the interest rate has risen higher.
While there are several lengths of rate lock periods (from 15 to 60 days), the longer ones are usually more expensive. The lender will agree to freeze an interest rate and points for a longer span of time, like 60 days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of fewer days.
There are more ways to get a reduced rate, in addition to opting for a shorter rate lock period. A larger down payment will result in a reduced interest rate, because you'll be starting out with more equity. You might opt to pay points to lower your interest rate for the loan term, meaning you pay more up front. For a lot of people, this is a good option..
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