TestFlight is Apple's service for beta testing, which means giving a test version of an app to a group of people before the public release. It works for apps, games and App Clips on Apple platforms, and it is part of App Store Connect, Apple's web portal for publishing apps.
The developer uploads a beta build to App Store Connect, writes what testers should test, and invites them. Internal testers are up to 100 members of the development team. External testers can be up to 10,000 people, invited by email or by a public link; their first build must be approved by Apple's App Review. Testers install the build with the TestFlight app, and the developer does not need to register their device IDs (UDIDs). Testers send screenshots with comments, and crash reports reach the developer in App Store Connect. A build can be tested for up to 90 days. When testing ends, the developer submits a tested build for App Store review.
TestFlight has no separate price. It is included in the Apple Developer Program, a membership with an annual fee.