Guide
A Document is a Rich Text view object that lives in the Sidebar within one of your Spaces.
Alternatively, Documents can be associated with an entity in a given Database (provided the Database has the Documents field enabled - see Fields ).
Check out Rich Text for more details.
If you want to build a wiki, but not sure whether to use Documents or Document databases, check this community topic.
You can disable Documents and they will be hidden from UI everywhere. Navigate to Settings → General and find Disable documents option.