A rate lock is your lender's written promise to honor a quoted interest rate for a set window — commonly 30 to 60 days — while your loan moves to closing. Without one, the rate on your loan estimate is just a snapshot: if the market moves before closing, your payment moves with it.
Why it matters: mortgage rates can shift daily, and even a small move changes a monthly payment you'll live with for decades. Locking turns a maybe into a fixed number you can budget around. The trade-offs: longer locks cost a bit more, and if rates fall after you lock, you're stuck unless your lender offers a float-down option — usually for a fee.
A concrete mechanic: if your closing slips past the lock's expiration, extending it costs money, typically a small fraction of the loan amount per extra stretch of days. So lock when you're confident in your timeline. You can watch where rates are trending on the mortgage rates page before pulling the trigger.