Apple first announced at WWDC in June that app developers would have to ask users for permission in order to track and share their IDFA identifier for cross-property ad targeting purposes. The plan is to launch these changes in early spring, with a version of the feature coming in the next iOS 14 beta release.
The App Tracking Transparency feature moves from the old method where you had to opt-out of sharing your Identifier for Advertisers (IDFA) to an opt-in model. This means that every app will have to ask you up front whether it is ok for them to share your IDFA with third parties including networks or data brokers.