Interest Rate 101
When you get a mortgage, you are charged an interest rate. This is the rate, which the lender charges you for using their money to buy a home. It determines how much your monthly payments will be. Generally speaking, the higher the interest rate, the higher your monthly payment.
Mortgage interest rates change constantly, daily, even hourly. If you speak to a lender and are quoted a specific interest rate, that's not to say you'll necessarily get that rate when you close on your loan. Not unless you formally lock-in that rate with the lender. Locking in an interest rate will guarantee you get your loan with a particular interest rate. Lenders will allow you to lock in for 15, 45 or 60 days. But the longer you lock in, the more expensive it will be, since it's more of a risk to lenders.
-Courtesy of Yahoo Real Estate