The content plugin is part of the core plugins that you cannot remove. It handles the content, what you see as a page.

Actually, the content plugin handles:

• pages
• branches (various major versions of a page)
• revisions (various minor versions of a page)
• secret data
• caches
• attachment, although the attachment plugin takes care of many of the features in link with attachments, the content plugin still handles the pages for attachments
• handling of plugin specific fields (read/write from the database)
• links with the help of the links plugin

The content plugin is very low level and as such it does not even itself add content to the database, even though it implements many functions to handle the feat... On setup, the output plugin is actually in charge of that work for the content and a few other low level plugins.