Extension:Description2

The Description2 extension adds a description meta tag for articles automatically based on the content. This functionality appears to have been removed from MediaWiki at some point. There are some situations where description extraction (by entities less sophisticated than Google, like Facebook) will extract the wrong description information from the wiki and use something like the sitenotice as the description for a page instead of its content. This extension also adds the description to the parser output in a way that other extensions can make use of (to say provide description in a different format like Extension:OpenGraphMeta does, or perhaps to use it in article lists on the wiki).

Extra options
If you set  in your LocalSettings.php a tag and a  parser function can be used to control the description outputted for the page and override it with a custom one. You can use these parser extensions like so: This is a description 

They all do the same thing, is supported so that Description2 can be used as a replacement for Extension:MetaDescriptionTag. The content="" feature was an extra added since it didn't take much to add and was described that way on Extension:MetaDescriptionTag's documentation page even though it appears merely to be a mistake in the examples. You can use the description2 parser function instead of using #tag to provide a description tag you can control with a template's variables.