Organizations thank volunteers in different ways: a weekend pass, meal tickets, a shirt, camping. Whatever yours offers shows up on your account.
How you earn one
Each perk has a rule behind it — a number of hours, a number of shifts, working a particular crew or a particular weekend. When you meet the rule the perk appears on your page.
It appears when the hours are credited, which is not the moment you finish the shift. If you have just worked the shift that got you over the line, give it until the day after before concluding anything is wrong.
Collecting it
Take your badge to wherever the organization says — usually the same tent you check in at. Whoever is on the tablet marks it collected.
A perk can only be collected once. That is the point of recording it, and it is why the answer to "can I just grab another one" is no, even when the person asking is lovely.
If you think you have earned something that is not showing
Check your hours first: perks are calculated from what has been credited, so an hours problem looks exactly like a perk problem. Your hours covers how to get that corrected. If the hours are right and the perk still is not there, ask your coordinator — the rule may not say quite what you think it says, and they can see it.