Definition of check in phrasal verb from the Oxford Advanced Learner's Dictionary

Phrasal Verbs
 to go to a desk in a hotel, an airport, etc. and tell an official there that you have arrived Please check in at least an hour before departure. We've checked in at the hotel. related noun check-in See related entries: Staying in a hotel, Plane travel
 to leave bags or cases with an official to be put on a plane or train We checked in our luggage and went through to the departure lounge. related noun check-in See related entries: Plane travel, Airports
See the Oxford Advanced American Dictionary entry: check in