TestFlight is an beta testing app/service offered by Apple, which you can download apps from the app developer and use it for FREE.
Only 2 steps are needed:
Step 1: Download TestFlight from App Store on your iPhone/iPad.
This will only be needed once. If you already have one, please skip this step and go Step 2.
Step 2: Open the email which the developer sent to you. One example of this kind of email should looks like:
View this email on your iPhone/iPad, click the View in TestFlight, it will automatically open up the TestFlight app, and click Accept. DONE~