Filters are part of the Semantic Drilldown extension used on Semantic MediaWiki-powered sites. According to the MediaWiki page, "filters based on semantic properties can be manually created and added to any top-level category." When the line SHOWINDRILLDOWN (enclosed on either side by two underscores) is used in categories, they are displayed on Special:BrowseData.
Pages in category ‘Filters’
The following 20 pages are in this category, out of 20 total.