After you select "Add Application", you will be taken to the page here:
From here you can fill in the fields for the App Name, BundleID, and Platform. Future uploaded builds will be grouped within this app. Once you enter this information in and select 'Save' you will be taken to the App's App Token & SDK page where you can copy the App Token to put into your build (if using the TestFlight SDK).
NOTE: Apps using this AppToken, have to match the same BundleID or an error will be received.
To learn more about Apps, please see the article here
To learn more about App Tokens, please see the article here