This is a tracking category.It is used to build and maintain lists of pages—primarily for the sake of the lists themselves and their use in article and category maintenance. It is not part of the encyclopedia's categorization scheme.
This category is populated based on the parameter at MediaWiki:template-loop-category.
When the MediaWiki parser detects and stops a loop in a template, it displays the message MediaWiki:Parser-template-loop-warning. That message used to categorize any page that displays it into Category:Template loop warnings which was a subset of this category.
For a list of MediaWiki-populated tracking categories, see Special:TrackingCategories.