Apple Advertising Identifier is a single value that is tied to the user’s phone (or other Apple devices) and can be accessed by mobile applications. It enables app publishers and marketers to track and target their audiences on iOS 6 (and beyond) devices.
Advertisers can track behaviors and users’ interests across app networks or platforms like it is done by browser cookies for web surfing.
Apple advertising identifier has replaced the Unique Device Identifier (UDID) which had been criticized by Congress and privacy advocates. Unlike UDIDs, the Apple advertising identifier gives consumers the ability to opt-out from targeted ads. Users can also reset the identifier.
For more details, see Apple’s IFA.