In one sentence
An agreement fixing the loan's interest rate for a set period, protecting the borrower from rate movement between commitment and closing.
The longer version
Rate locks matter most on long fixed-rate loans where a small rate move is meaningful money. The lock has a term, a cost, and usually breakage consequences if the loan does not close in time.
The lock date puts a hard deadline on the closing, which makes it one of the calendar's fixed points and a reason diligence delays get expensive.
Keep reading