Hi Paola
Great question.
When a user purchases a ticket, the ticket / email will contain a QR code. The user needs to show that QR code when entering the event and someone at the entry will need to scan it with a QR code scanning app.
The person scanning will need to be logged in to the site selling the tickets. When the code is scanned it will mark the owner of the ticket checked in.
Hope this helps. Let me know if you have further questions.
Cheers,
Andras