It is a authentication plugin known as Simpleauth. Most MCPE server have this plugin to prevent people from impersonating others. It is a simple login system.
When you first join the server, you must do /register password - Replace passwordwith your own password
After you do this, and you are returning to the server, you need to do /login password - Replace passwordwith the password you used on /register
When you first join the server, you must do /register password - Replace password with your own password
After you do this, and you are returning to the server, you need to do /login password - Replace password with the password you used on /register