To check in is to report your arrival at a hotel desk or an airport counter. Check in online.
Airlines want you to check in early because it tells them who is actually coming. To check in on somebody is a different and kinder use of the same phrase.
1phrasecheck inTo report your arrival at a hotel desk or an airport counter.
“You can check in from three o'clock.”
check in is an action.
Practised in Everyday Verbs