Check-ins
Checking in marks that a registered hacker actually attended. A check-in moves a registration from registered to completed.
Self check-in (hackers)
If your event has Allow self check-in turned on, hackers can check themselves in from the event page once the event is pending or in progress. If you've set a check-in window, self check-in opens a set number of hours before the event starts.
Manual check-in (organizers)
You can also check a hacker in yourself from their registration on the Registrations page — unless your event type has organizer check-in disabled.
Why check-ins matter
Only checked-in hackers count as attendees. For GHW, check-ins into mini events count toward bonus swag only if the mini event's parent is set to the main GHW event — see GHW event setup.