Manual:$wgFileCacheDirectory

Directory where the cached page will be saved when using file caching.

Details
If you switch on server-side file caching $wgUseFileCache = true;, then you can use this variable to specify a where the cached pages will be saved on your server filesystem.

It's an absolute path, so you could specify somewhere under /tmp for example: $wgFileCacheDirectory = "/tmp/yourcache";

...but you can specify relative to other directory path variables e.g.: $wgFileCacheDirectory = "{$wgUploadDirectory}/cache"

This specifies a "cache" directory under the image uploads directory (normally called "images"). This is the default setting in fact.

You must create the directory and ensure it is writeable (by the php user). This is even true if you leave it as the default (You must create the "images/cache" directory).

See also meta:Help:File cache