The second column of icons in the attendee list is for onsite management purposes.
If the check-in icon (most left) is red, the person didn’t enter the event yet. A green icon means the person entered the event. If a person was registered more than once, this warning icon shows:
You can also easily filter your attendees based on their status during the event. If you want to see which attendees are already present at the event, you can filter on ‘Present’. If you want to see all attendees that aren’t present at the event yet, you can filter on ‘Expected’. The filter ‘Quitted’ shows the attendees that were present at the event, but already left.