A slug in SEO is the part of the URL of a webpage. Let's say for example at the top of this page you'll find URL of this page like this https://www.grootsolutions.com/blog/what-is-slug-in-seo. This is the complete URL but when you look end of the URL ("what-is-slug-seo") this part is called a slug.
The terminology is mostly used in WordPress to make any change to the URL while writing a blog post or while creating a page.
In WordPress, while writing a blog post or creating a page you can only edit the slug of the post not the entire URL of the post. The editable part has bold font compared to rest of the URL font.