http://nksecur5aoxbew7x4t2jebk7ordomil3a4xubeamno76zexicppwhiad.onion/how-to-pgp.html
Any unencrypted messages could be read by either administrators, LEA owners (if they took it over) etc. Each person has two keys, a public key and a private key: Your public key is shared and can be used by others to encrypt a message to you, that you can decrypt with your private key.