Courtesy of a finance blog I read regularly, Dan Green at The Mortgage Reports. He explains mortgage rate locks better, and quicker than anyone I’ve ever encountered.
A Rate Lock Commitment is a bank's promise to honor a specific mortgage rate for a specific period of time. It's a contract, of sorts, in which the lender says: "Provided you close on your loan in the next however-many days, we'll make sure you get your locked rate."
Want more information? Lots more? Visit the Consumer’s Guide to Mortgage Lock-Ins by HSH Financial Publishers. In addition, the Federal Reserve publishes a pamphlet about mortgage rate locks.