Google Chrome (and Chromium, Microsoft Edge, etc.) browsers now track which websites users visit, and group users into "cohorts" of people who visit similar sites, so that Google can serve them targeted advertising based on their browsing history. See https://blog.malwarebytes.com/cybercrime/privacy/2021/04/millions-of-chrome-users-quietly-added-to-googles-floc-pilot/ and https://spreadprivacy.com/block-floc-with-duckduckgo/.
Site owners must opt-out of this behavior if they don't want visits to their site to be tracked, by sending the HTTP response header Permissions-Policy: interest-cohort=(). See https://github.com/wicg/floc#opting-out-of-computation.
Allowing visits to be tracked via FLoC is a privacy risk to our users. Although the resulting cohort IDs are meant to group users anonymously, it is possible to de-anonymize visitors using them. See e.g. https://github.com/WICG/floc/issues/100.