What does it mean to lock the interest rate?

Mortgage rates can change from the day you apply for a loan to the day you close the transaction. If interest rates rise sharply during the application process it can increase the borrower’s mortgage payment unexpectedly. Therefore, a lender can allow the borrower to "lock-in" the loan’s interest rate guaranteeing that rate for a specified time period, often 30-60 days, sometimes for a fee.