What is Check in ?
Check in is (verb) 1. (at a hotel.) to arrive at a hotel and sign for a room He checked in at 12.15. We checked into our hotel and then went on a tour of the town. 2. (at an airport) to give in your ticket to show you are ready to take the flight Please check in two hours before your departure time. 3. to check baggage in to pass your baggage to the airline staff to put it on the plane for you My bag hasn’t been checked in yet.
source: Easier English, Student Dictionary Upper Intermediate Level