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.
This video about passkeys is fascinating. They are very secure even if your pin is 1234. The only way for someone to hack your account is if they have your device.