A slug is a few words that describe a post or a page. Slugs are usually a URL friendly version of the post title (which has been automatically generated by WordPress), but a slug can be anything you like. Slugs are meant to be used with permalinks as they help describe what the content at the URL is.
Example post permalink: https://www.wordpresshustler.com/glossary/slug/
The slug for that post is “slug”.
Slugs are also used for post types, categories, tags, etc. Like post and page slugs, this is usually for creating URL friendly versions of these items.