IT'S SIMPLE. Developers need to find those last minute bugs before launch. Testers get to be the first to try out those latest and greatest features.
Developers - simply fill out the form with your app name, app icon, description, test details and the public beta link.
Testers - search for your beta of choice, enter your email, and click the Public Beta link, where you will be instructed to install TestFlight and download the beta app. (Make sure to click the link from your mobile device).
Then let the magic begin!