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.
Me, at the bank:
Robbers, as they enter the bank: everybody freeze
Me: ah shit
Robbers: everyone give me your phones
Me: aw hell naw
mission impossible style shootout