Apparently it means you're not gonna show up on their Trending page/tab.

Everything else should work as normal, your posts aren't hidden, nor replies or anything like that (apart, again, from the trending page/tab). Proof of this is I'm on Primal web and can see your content with no issues.

Also it seems to be a temporary solution as they are working on tools to add content warning to posts and opt-out filtering options.