What does it mean to lock the interest rate?

Due to the nature of interest rate movements, mortgage rates can change dramatically from the day you apply for a mortgage loan to the day you close the transaction. If interest rates rise sharply during the application process, it could make a borrower's mortgage payment larger than he/she previously thought. To protect against this uncertainty, a lender can allow the borrower to 'lock-in' the loan's interest rate, guaranteeing the borrower the prevailing loan rate for a specified period of time (often 30-60 days). A lender may or may not charge a fee for this service.