A Wikipedia admin is an unpaid volunteer (not a staffer) with access to extra functions of the MediaWiki software Wikipedia uses. They typically get these rights through a community vote like this one: https://en.wikipedia.org/wiki/Wikipedia:Requests_for_adminsh...
"Administrators, commonly known as admins or sysops (system operators), are Wikipedia editors who have been granted the technical ability to perform certain special actions on the English Wikipedia. These include the ability to block and unblock user accounts, IP addresses, and IP ranges from editing, edit fully protected pages, protect and unprotect pages from editing, delete and undelete pages, rename pages without restriction, and use certain other tools."
For more info see: https://en.wikipedia.org/wiki/Wikipedia:Administrators