Manual:$wgCacheDirectory/en

Details
This directory stores some cache files specific for a given wiki, which means wiki farms should have one such directory per wiki. Should not be accessible from the web (chmod 700 on Unix). If  is set to false (or not set), the wiki will be slower.

The proposed value is. This is commented in the file LocalSettings.php created during the installation and can be de-commented. In this case, you should create the directory  in your MediaWiki directory and make it writable by the web server.

Note that you should not set this directory to  else there could be conflicts with other MediaWiki installations on the same server (wiki farms or shared servers).