The administrators guide is for configuring global settings, top-level user management and plugin setup. Plugins are optional modules, which provides specialized functionality.
It is important to understand the setup options/features, to make sure Opinio runs as intended. Some settings must be set, others are optional. For some of the settings, default values will often work OK, but since Opinio runs in a wide variety of environments, it is impossible to have a common setup scheme that fits all.