What is a "rate lock period"? How can you make sure your rate is low?

A rate lock or a rate commitment is a lender's promise to hold a certain interest rate for you for a specified period of time while your application is processed. This prevents you from going through your whole application process and at the end of it finding out the interest rate has gone up.

A rate lock period can vary in length, and longer ones usually cost more. A lender will agree to "hold" your interest rate and points for a longer period, say 60 days, but in exchange the rate may be higher than with a shorter rate lock period, for example.

There are many ways besides opting for a shorter rate lock period to get a lower rate, though. A larger down payment will result in a lower interest rate than a smaller one, because you're starting out with more equity. You can pay points to lower your rate over the life of the loan, but that means you pay more up front. For many people, this makes sense and is a good deal.

Closing costs are fees such as appraisal, escrow, title insurance, etc. which are necessary for you to close the loan. 

Finally, the interest rate a lender is willing to offer you depends on your credit score and your income-to-debt ratio. If you have good credit and your income far exceeds your debt obligations, you will qualify for a lower rate.