Slug is used to make a name that is not 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 in general case slug is a more appropriate combination of other 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'?
