A lock-in, also called a rate-lock or rate commitment, is a lender’s promise to hold a certain interest rate and a certain number of points for you, usually for a specified period of time, while your loan application is processed (Points are additional charges imposed by the lender that are usually prepaid by the consumer at settlement but can sometimes be financed by adding them to the mortgage amount). Read more on the Federal Reserve's website.
A Consumer's Guide to Mortgage Lock-Ins (Federal Reserve Board)