What does it mean to "lock a loan"?

What does a “loan lock” mean? One top agent sent out a note to her staff. “I think as a consumer, or even a loan officer, when we lock a loan, we feel like we are simply "securing" or "holding" that rate for a client. That is only part of it. Once a lock is made, at that moment, the investor is expecting delivery of that loan at the interest rate as part of their portfolio. (In essence, the loan might not be closed, but it is already sold.) If you can't deliver, or don't close on time, or you are just simply "trying" to secure a "deal" based on rate, then the investor is going to call your lender and ask, “Where is my loan? Where is my money?" Then your lender might try to "replace" that loan with another loan, or just say to the investor, "Sorry." You are not just simply holding for you and your client an "Insurance Policy" to try to get that rate, if by some chance you get the loan, you are, in fact, impacting the investors who are trying to make money on those sold loans. It may be hard to miss that "single day" rates are awesome...but, if you are not in Contract, and you don't have an Appraisal...and you don't have a true file you can close in 30 days...then DON'T LOCK...UNTIL YOU DO! LOCK when you KNOW you are going to close it. Lock AFTER you have an approval. Don't lock at multiple Banks. A lock is a promise to deliver!”