Slug is a newspaper term. A slug is a short label for something, containing only letters, numbers, underscores or hyphens. They’re generally used in URLs. (as in Django docs)
A slug field in Django is used to store and generate valid URLs for your dynamically created web pages.
Just like the way you added this question on Stack Overflow and a dynamic page was generated and when you see in the address bar you will see your question title with "-" in place of the spaces. That's exactly the job of a slug field.
The title entered by you was something like this -> What is a “slug” in Django?
On storing it into a slug field it becomes "what-is-a-slug-in-django" (see URL of this page)