A rate "lock" or "commitment" is a promise from the lender to lock in a specific interest rate and a particular number of points for you for a certain period while your application is processed. This protects you from getting through your whole application process and learning at the end that your interest rate has gone up.
While there can be a choice of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. A lender may agree to lock in an interest rate and points for a longer period, like 60 days, but in exchange, the rate (and sometimes points) will be more than that of a rate lock of a shorter period.
In addition to going with a shorter rate lock period, there are more ways you can attain the lowest rate. A bigger down payment will give you a reduced interest rate, because you'll be starting out with a good deal of equity. You might opt to pay points to reduce your interest rate for the loan term, meaning you pay more initially. For many people, this is a good option..
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