Hi @Michaela43,
Welcome to the Community Center!
Guests don't need to do anything to check-in, Airbnb will assume that they checked in at the check-in time. I presume the app is just reminding them that they can now check-in (i.e. arrive and get the keys from you).
Just to let you know, I have moved your post to a new thread here in Community Help, so that it doesn't get lost in an old thread. It will be easier for others to find it here and offer advice! I have also given it a title, please let me know if you would like to change it!
Kirstie