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.
Interesting, I'll probably just have to wait till either Bitwarden supports Passkeys, or wait till Firefox on Linux supports cross-device Passkeys (so, my phone for example) as yeah a 25 key limit is not likely to be worth purchasing an upgrade for just yet.