If you do not show up and check yourself into the event via the mobile app, you will be charged a no-show fee (typically $10 but varies per event) for each of of the tickets you've claimed.
For most events, you are allowed to cancel your ticket up to 12 hours prior to the event.
Perks work a little bit differently. A Perk is kind of like an event that lasts a week. As long as you redeem it any time during that week, you will not be charged a no-show.
IMPORTANT: If you cancelled your ticket and got the cancellation email, but still receive notifications about being charged, you can ignore them. Sometimes the system sends them out if people cancel after a certain point.