IDFA is the abbreviation for IDentifier For Advertisers. It’s an Apple-created random identifier assigned to a user’s device.
Advertisers use it much like a cookie on the web: it is a marketing measurement technology that enables an advertiser to measure when a user of a phone has taken actions like clicking on their ad, installing their app, and connecting them in order to analyze the user journey.
In iOS 14.5, Apple made the IDFA opt-in, after years of making the IDFA available by default unless someone explicitly opted out. Only about 20% of people opt in to sharing their IDFAs via Apple’s ATT (App Tracking Transparency) prompt.
The IDFA can be reset by users manually, or by default, if people opt out of using it on all installed apps, so it’s not a permanent identifier. The IDFA replaced the UDID, a hard-coded device identifier. The equivalent device identifier on Android is GAID, Google’s ad identifier.