A rate "lock" or "commitment" is a promise from the lender to lock in a particular interest rate and a certain number of points for you for a specified period of time during your application process. This keeps you from working through your whole application process and discovering at the end that your interest rate has gotten higher.
While there are various lengths of rate lock periods (from 15 to 60 days), the longer ones are generally more expensive. The lender will agree to hold an interest rate and points for a longer period, like sixty days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of a shorter period.
In addition to going with a shorter lock period, there are other ways you can score the best rate. A bigger down payment will result in a lower interest rate, because you'll have a good deal of equity at the start. You could choose to pay points to reduce your interest rate over the loan term, meaning you pay more up front. One strategy that makes financial sense for many people is to pay points to bring the rate down over the life of the loan. You'll pay more up front, but you'll come out ahead, especially if you don't refinance early.
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