A system for websites to request a proof of the "integrity" of a user's browser and underlying OS/hardware, and "attesters" to check this "integrity" and provide the proof. If that sounds vague, that's because it is. What "integrity" means is for the "attester" to decide.

Google would of course be one of the major "attesters", and could just deny the proof if you installed an ad blocker or VPN for example. In this case you would likely not be able to access the website anymore, because your device is deemed as "untrustworthy".

So it's a way for big companies to decide who can still use big parts of the internet and who can't, based on whether it would make them money.