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 will not protect you against the government.
As I said elsewhere, it's for web services. The web service provider will have to surrender your data on a court order. They can decouple it from the passkey. The passkey doesn't encrypt your data at the provider, it's only used for authentication.
If you really want to protect your data, you will need to use a different encryption solution. Something like full disk encryption with a complex pass phrase.