Google Topics will track your browsing and divvy it up into 300 categories for advertising. It replaces Federated Learning of Cohorts (FLoC).
When you hit upon a site that supports the Topics API for ad purposes, the browser will share three topics you are interested in — one for each of the three last weeks — selected randomly from your top five topics of each week. The site can then share this with its advertising partners to decide which ads to show you. Ideally, this would make for a more private method of deciding which ad to show you — and Google notes that it also provides users with far greater control and transparency than what’s currently the standard. Users will be able to review and remove topics from their lists — and turn off the entire Topics API, too.