When you're offered a "rate lock" from your lender, it means that you are guaranteed to keep a particular interest rate for a certain number of days while you work on your application process. This means your interest rate can't get higher during the application process.
While there can be a choice of rate lock periods (from 15 to 60 days), the longer ones are typically more expensive. You can get a longer period for your lock, but in doing so, will likely have a higher rate than you would with a shorter span of time
There are other ways to get a better rate, besides going with a shorter rate lock period. A bigger down payment will result in a better interest rate, since you are starting out with more equity. You might choose to pay points to improve your rate over the life of the loan, meaning you pay more initially. To a lot of people, this is a good option..
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