Hi Morgan,
A "check in" simply means the arrival of a guest. It is up to you to decide if you will welcome them personally, or maybe you have left the key for them somewhere. You might show them around and explain a bit about the apartment/room, the area, etc.
There is no need to notify Airbnb of anything or formally register the check in online. Unless there is any trouble, they will just asume everything went well and your guests have arrived. 🙂
I hope this helps?
Best,
Lilli
