This means if you're under 13 you can watch videos on YouTube, but you can't create an account or use notifications or subscriptions.
THis does create weird things though. Children like that notification bell, and so they use their parent's accounts. This means advertisers think the person viewing the videos is over 18, and that it's okay to show ads for alcohol or gambling.
That's not ok in the UK, and the regulator tells advertisers to be more careful when placing ads.