This addon is a plugin. You need to extract it to your /plugins folder to install it.
Adds SmartyPants Typographer support to discussions and comments.
SmartyPants: translates "plain" ASCII punctuation characters into “smart” typographic punctuation HTML entities.
Typographer: is an extension to SmartyPants adding support for different quotation marks as well as for converting normal spaces to unbreakable spaces in some circumstances, thus preventing inappropriate line breaks.
Basically, this plugin makes punctuation look as pleasing as it can do.