What is mortgage rate lock-in and why does it matter?

Mortgage rate lock-in means a lender guarantees a specific interest rate for a set window of time while a loan moves through underwriting, protecting a borrower from a rate increase before closing. It matters because rates can move daily, and without a lock a borrower risks closing at a higher rate than the one quoted when they applied. Locks typically carry an expiration tied to the expected closing date, so a delayed closing can require an extension, sometimes at a cost. A lender can explain the specific lock terms and any extension policy before a buyer commits to a closing date.