The ParserFunctions extension enhances the wikitext parser with helpful functions, mostly related to logic and string-handling. Since MediaWiki 1.15, ParserFunctions has incorporated most (but not all) of the functions from the StringFunctions extension, which may be enabled or disabled (see install instructions hereafter).
This extension is bundled with MediaWiki 1.18 and above. Thus you do not have do download and extract the extension as described. However, you still need to follow the other instructions provided.
Download and extract the files in a directory called ParserFunctions in your extensions/ folder. If you're a developer and this extension is in a Git repository, then instead you should clone the repository using:
This extension is being used on one or more Wikimedia projects. This probably means that the extension is stable and works well enough to be used by such high-traffic websites. Look for this extension's name in Wikimedia's CommonSettings.php and InitialiseSettings.php configuration files to see where it's installed. A full list of the extensions installed on a particular wiki can be seen on the wiki's Special:Version page.