According to Google, “When Apple’s policy goes into effect, we will no longer use information (such as IDFA) that falls under ATT for the handful of our iOS apps that currently use it for advertising purposes. As such, we will not show the ATT prompt on those apps, in line with Apple’s guidance.”
Prior to these changes, apps on iOS were able to collect these advertising identifiers which allowed them to create ads that were more targeted and personalized to the user. Some have viewed this as being an invasion of privacy, and Apple, who has touted their commitment to user privacy, decided to do something about it.
Google does warn app developers that this could potentially have a negative impact on their ad revenue, but exactly by how much remains to be seen. Apple has also introduced other changes like requiring developers to include privacy labels in their App Store listings, something that Google has yet to do.