FLoC - Next Generation Internet User Tracking
What is FLoC? Google is looking to change the current advertising standard of using 3rd party cookies to keep track of you and are proposing the Federated Learning of Cohorts (FLoC). 0.5% of random Chome browser users are currently trialling this, without knowing. Their browser's have been randomly set to opt-in to test Google's new tracking feature.
The EFF have written a great explaination. And you can test if your browser is currently participating. If you run a website you can find instructions on how to opt your website out.