The Identifier for Advertisers (IDFA) is a unique identifier assigned to each device running Apple's iOS operating system. The IDFA is used by advertisers to track the performance of their mobile advertising campaigns on iOS devices.

The IDFA is a randomly generated string of characters that is assigned to a device when it is first used. Advertisers can use the IDFA to track the number of impressions, clicks, and conversions generated by their ads on iOS devices. The IDFA also allows advertisers to target specific audiences and to measure the effectiveness of their mobile ad campaigns.

Apple has implemented strict privacy controls for the use of the IDFA. iOS users must opt in to being tracked by the IDFA, and they can opt out at any time by going to the privacy settings on their device. Apple has also introduced new privacy controls in iOS 14 that require apps to ask users for their permission before tracking their IDFA. These changes have raised concerns among advertisers about the future of mobile advertising on iOS devices.