The most common definition of slug, as it pertains to CodeIgniter, is a string that references a record (or post) in something like a CMS (Content Management System…think Wordpress or PyroCMS.)
The purpose is to build a URI that is used as a substitute for a database ID. For example, consider the following URLs:
The latter uses a “slug” (bretticus-explains-slugs) that can be used to search a database story record with a corresponding field value, instead of using the story record’s id field (123.) Slugs, by my definition, are popular for user-friendliness, style and SEO optimization.