A slug is an alternative to a name that would otherwise not be acceptable for various reasons – e.g. containing special characters, too long, mixed-case, etc. – appropriate for the target usage. What target usage means is context dependent, but could include usage in a URL or name of a file or database table for example. In the general case a slug may be composed from a combination of multiple fields; in the above case, only one field is used – title.
Have a look at these SOqs, too:
- What is a “slug” in Django?
- Replacing %20 with a Dash
If you are into the origin of the term, see this:
- What is the etymology of ‘slug’?