The Content Access module is a contributed module that allows a permitted user the ability to set fine-grained access control to content.
This means that content contributors can determine whether or not to allow the visibility of published content.
Using this module, content access can either be broadly restricted by content type, which is configured by an advanced author or administrator. It can also be configured so that content can be controlled on a per node basis. When this option is used, a tab is provided on each node that allows the content contributor to restrict the content as needed. This means that authors can restrict content as well.
The most common use case for a feature such as this is to create something like an "intranet" within a Drupal site whereby content can be seen by members of the site only.