With the 5 to 7 word sentence, I assume they mean the new password you have to enter. They want to bring you to use a strong password if you access your rig from the Internet. If you are behind a router or firewall in a private LAN and don't want have access from outside it doesn't matter.

Admin account is the username you used to log in into the webinterface. Normally it was "admin".

Management Network: If you access only from within your local LAN, leave it to "LAN". Otherwise please explain your situation first.

Edit: I think it will also change your root password (didn't try it respectively entred my old password). AFAIK if you change your root password in the shell with "passwd" it will not change your "admin" password for the miner website. If you change the "admin" password in the miners website, it will also change your root password.