This category tracks pages that have had the __INDEX__ magic word added to them, so that they show up in web search engines such as Google. This category is added automatically by the MediaWiki software. The category name is defined in MediaWiki:Index-category.
Please note that adding __INDEX__ to a page will not immediately add it to search engine results. It may take up to 24 hours for search engines to clear their caches and pick up on the change.