When you're promised a "rate lock" from the lender, it means that you are guaranteed to get a certain interest rate over a determined period for your application process. This keeps you from going through your whole application process and discovering at the end that your interest rate has gotten higher.
Rate lock periods can be various lengths of time, anywhere from 15 to 60 days, with the longer period usually costing more. A lender will agree to freeze an interest rate and points for a longer span of time, such as 60 days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of fewer days.
In addition to going with the shorter lock period, there are other ways you are able to get the lowest rate. A larger down payment will give you a better interest rate, because you'll be starting out with a good deal of equity. You can pay points to reduce your interest rate over the life of the loan, meaning you pay more initially. 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 will pay more up front, but you'll save money in the end.
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