The general settings include options that don't require their own link in the sidebar. Most of these are items that will change the interface or user experience.
This name will be displayed to users when they select a server, and clients might use it in other locations as well. The default value will be the hostname of the computer.
This message will be shown to users when they login on your server.
You can add custom CSS rules for minor changes to the web interface that don't require their own theme.