According to this WikiHow article, one of tools needed to change a spark plug is "an extension." Is this a type of wrench?
The tool referred to is an accessory for a ratchet/socket wrench setup. It simply allows one to access fasteners that are inside deep holes.
It has a female square drive on one end--this connects to the ratchet; and a male square drive on the other end--this connects to the socket.
Extensions of various lengths
Ratchet with Extension and Socket in place