When you're promised a "rate lock" from the lender, it means that you are guaranteed to keep a certain interest rate over a determined period while you work on the application process. This means your interest rate can't grow as you are working through the application process.
Rate lock periods can vary in length, between 15 to 60 days, with the longer spans generally costing more. You can get a longer period for your lock, but in doing so, will probably have a higher interest rate than you would with a shorter rate lock span of time
In addition to choosing a shorter lock period, there are more ways you may be able to score the lowest rate. The larger down payment you make, the better your interest rate will be, as you will be entering the loan with more equity. You can pay points to improve your rate over the loan term, meaning you pay more up front. One strategy that is a good option for some is to pay points to bring the rate down over the term of the loan. You pay more up front, but you'll come out ahead, especially if you keep the loan for a long time.
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