To check into your Partiful event, when you arrive, show your QR code to the host or check in staff to be scanned.
You can find your ticket by navigating to the event page → click View ticket near the bottom of your screen.
This will pull up your ticket and QR code. The host will scan your QR code to check you in. The host can also check you in manually by searching your name. If you bought multiple tickets, all your tickets will be checked in at once (learn more here).