Freezing the Rate
When you are offered a "rate lock" from the lender, it means that you are guaranteed to get a certain interest rate for a certain number of days while you work on your application process. This means your interest rate can't go up as you are going through the application process.
Although there may be a choice of rate lock periods (from 15 to 60 days), the longer spans are generally more expensive. You can get a longer period for your lock, but in doing so, will likely have a higher rate than you would have with a shorter rate lock span of time
More Ways to Get a Great Interest Rate
There are other ways to get a low rate, besides choosing a shorter rate lock period. The larger down payment you can make, the lower your interest rate will be, because you will be entering the loan with more equity. You could choose to pay points to improve your rate for the term of the loan, meaning you pay more up front. For many people, this makes financial sense.