The phrase "to lock in" means to secure or guarantee something, usually a favorable rate or condition. It can be used in various contexts such as financial transactions, contract negotiations, and personal plans. For example, when referring to interest rates, it means securing a low interest rate for a loan or investment by committing to the agreement immediately. In another context, it could mean finalizing arrangements for an event or trip, ensuring that the dates, locations, and other details are set in stone and cannot be changed. The phrase emphasizes the importance of acting quickly and decisively to take advantage of a good opportunity before it is too late.