DokuWiki uses a background process triggered by a webbug to execute administrative tasks like building the search index, creating a sitemap or updating metadata.
The indexer is located in lib/exe/indexer.php and is included as hidden image by the tpl_indexerWebBug() function.
For more info about the technique used see Web bugs for job scheduling
If you have problems with the indexer or the services it should provide, you can get some debugging output, by calling it directly and appending the parameter debug=1. You need to have the allowdebug option enabled.
Example: http://example.com/dokuwiki/lib/exe/indexer.php?id=start&debug=1