1.
When you check in or check into a hotel or clinic, or if someone checks you in , you arrive and go through the necessary procedures before you stay there.
I’ll ring the hotel. I’ll tell them we’ll check in tomorrow...
He has checked into an alcohol treatment centre...
Check us in at the hotel and wait for my call.
= register
≠ check out
PHRASAL VERB : V P , V P n , V n P
2.
When you check in at an airport, you arrive and show your ticket before going on a flight.
He had checked in at Amsterdam’s Schiphol airport for a flight to Manchester.
PHRASAL VERB : V P
see also check-in , check 6