In the movies, the really secure safes require two separate keys turned at the same time. The nuclear launch codes or the bank vaults have at least two separate locks. Take the famous heist scene in the first "Mission: Impossible" movie: the door to that super-secure room required a voice code, a key card and a retina scan.
It's not just the movies. Using multiple keys simultaneously is good security sense. Any one key can be stolen or compromised, but thieves still can't get in. When it comes to your personal information, you can't be too careful. That's why more and more big tech companies are stressing two-factor authentication.
That just means that you would use two ways of confirming your identity to access your accounts. If you bank online, it's almost guaranteed that your bank uses two-factor authentication. That's what Google wants to do also, but it has a very interesting way of going about it.
It's an ancient invention, used for centuries - but modernized to secure you digital life. It is a physical key.
Google wants you to start using a physical key in addition to entering your password to access your account. This security key would also be able to verify that the site you're accessing is a trusted Google gateway.