At the highest level, an event hashtag gives you the ability to easily track social media activity (Twitter and Instagram, specifically) related to your event.
Update your event's hashtag directly from the Settings tab.
You can search directly on Twitter or Instagram for your hashtag and track activity there. Or, even cooler, you can pull those photos and tweets directly onto your event page.
Check out our Social Gallery feature on how to build photo galleries comprised entirely of the images taken at your event and posted to Twitter and Instagram.