App Tracking Transparency (ATT) is a software capability built by Apple that enables users to safeguard their privacy to some extent. It basically forces apps to explicitly ask for consent from users in order to track their activity on apps and websites owned by other companies. If the user decides to not allow the app to track their activities, the app won't be given access to the system advertising identifier (IDFA), which is essentially a unique identifier that does not reveal your personal data.
If apps are found to be tracking the user after they asked not to be tracked, Apple may remove them from the App Store. Additionally, apps are obligated to provide users full access to their features with or without tracking permission. Now, Meta is potentially facing a class-action lawsuit for circumventing ATT.
As highlighted by 9to5Mac, Meta could theoretically use its own trackers in the embedded web browser to track every interaction you have on a website, including ad clicks.
Interestingly, in response to the lawsuit, Meta has admitted that it does monitor browser activity in this way, but it has emphasized that it collects no user data. This is a rather fine line considering that it theoretically could gather this information if it wanted, so it will be interesting to see how this holds up in court.
While the proposal behind the lawsuit is to get it converted into a class-action, this is something that first has to be approved by a judge.
Meta had previously protested the availability of ATT, saying that it hurts small businesses who rely on targeted advertising as their revenue streams. It also criticized the idea as a whole, claiming that it gives Apple preferential access to its advertising network, whereas other companies need specific permissions. That said, Meta has been recently fined in South Korea for not respecting user consent.