Support was first offered in May, but now Google suggests it as the default choice.
Passkey is some sort of specific unique key to a device allowing to use a pin on a device instead of the password.
But which won't work on another device.
Now I don't know if that key can be stolen or not, or if it's really more secure or not, as people have really unsecure pins.
passkeys sounds good on paper and for most users on day to day stuff should improve their security. But the failure path is horrible and it happens at the worst case most of the time. If I have the keychain on the phone and lose it or is out of battery and usually happens that I need to access some service like email, then if the email provider starts forcing people to use passkeys or you only have that method on, then I'm locked out of the account and can't use email. This will happen for all other services that one may need to use on an emergency. Personally I don't like it.
“Keychain” is often used colloquially to refer to a piece of software that holds passwords and other secrets, which can include passkeys depending on the implementation.