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.
There’s more ways such as scanning a Qr code to establish a connection from the app to the computer, or by presenting a number on one device which must be entered on the other