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:Unresolved-property-category.
For a list of MediaWiki-populated tracking categories, see Special:TrackingCategories. From the description there, "This category lists pages that reference Wikidata properties that cannot be found neither by their property ID nor label [sic]."