Both your Mac and the other Apple device must use Underpass in order to chat.
The other device can be on the same Local Area Network (LAN) as your Mac, or they can both be on the internet. See Basic concepts for more details. To chat on a LAN, select "New Local Area Network (LAN) Chat". To chat on the internet, select "New Internet Chat".
When you select a new LAN or internet chat, a new chat window will open, and you are presented with the chat settings. The first step is to enter a password.
The same password must be used on both your Mac and the other device. If you choose to leave the password blank, then the password must be left blank on the other device too. For security, it is highly recommended that you select a password for your chats. See Passwords and encryption for more details about passwords.
The next step is determined by the type of chat: