Plugins are the parts of the app that only some groups need - a film catalogue, a book shelf, a shared cookbook, a glossary. The Plugins tab is where a group turns on the ones it uses.
Plugins in the group settings lists what is available to your group and lets you switch each one on or off. The list is not the full catalogue: only plugins the platform administrator has enabled show up here, so an empty list means nothing is available rather than nothing exists. Some plugins add a Configure link next to their entry once they are on.
Enable it
Switch the plugin on in the list and save. Its own screens appear in the admin menu, and its public pages become reachable on the group's site.
Configure it if it asks
A plugin with settings of its own shows a Configure link. That is where its per-group options live, not on this tab.
Give it content
A freshly enabled plugin is empty. A film catalogue with no films is a link to an empty page, so fill it before pointing members at it.
Pick the plugins that match your loop
A group that votes on what to read next wants the book and voting plugins together; a dinner club wants dishes. Turning on everything gives members a navigation bar full of empty pages, which reads as a site nobody maintains.