Manual:$wgAllowPageInfo

Details
This setting, if enabled, allows users to get some extra information about the page by specifying "action=info" in the URL.

For example, entering  might generate the following (fictional) output instead of the main page contents:

MediaWiki Information for page  Number of watchers: 23 Number of edits (content page): 895 Number of distinct authors (content page): 31 

Currently you have to add the action manually to the URL, as there is no link to this page regardless of the value of this setting. See bug 16362.

Note that on most (all?) Wikimedia servers (e.g. Wikipedia, Wiktionary, etc.) this function is disabled, as it is very inefficient and because it allows potential vandals an easy way to find unwatched targets.