If you want to learn what Floc is for the first time you probably shouldn't start with an article titled "Google’s FLoC Is a Terrible Idea". I also don't know what it is but will just wait for a more neutral source hopefully.
FLoC is Google's new way to target "cohorts" of users with advertising. The idea is that Google will classify each user into a cohort (and that classification will change over time as the user visits more web pages, and perhaps other data is acquired) and only that cohort is reported to the advertiser, which can then use it to serve appropriate ads.
On the flip side, this is obviously still targeted advertising, and some people have a strong negative outlook towards that idea in general. Also, it's been said that if you can manage to track a person across just a few cohort changes, you can personally identify them, which is contrary to the entire idea about FLoC protecting a person's privacy.
In short, proponents think it's better than tracking cookies, and opponents think it's still privacy invasion, and not much better than tracking cookies.
Or illegal. IANAL, of course, but there are certain cohorts (age, disability, etc.) which are illegal to discriminate against in the US. But since Google doesn't have insight into what a cohort describes, they can't ensure that cohorts are being handled properly according to the law.
I'm sure Google has a "get out of government oversight" card from their lawyers, but like biased AI, this seems like it's on the wrong side of "grey".
https://github.com/WICG/floc#excluding-sensitive-categories
I haven't seen details of how they expect this to actually work though.
Tracking, advertising, user cohorts does not fit into the "browsing" part. That might be enough to feel why "Google's FLoC is a Terrible Idea".