With TestFlight beta testing you can distribute builds of your app to testers, collect feedback, and prepare your app for release to customers. TestFlight beta testing is optional; you can submit your app for review without it. However, it’s easy to do, free, and a valuable way to improve your app before releasing it to customers. You can enable TestFlight beta testing for up to 100 apps at a time in your App Store Connect account.
For an overview, go to TestFlight Beta Testing.