Slug is the URL of a page or a department and is usually auto created using the title of the page/department. The slug can be changed or shortened after being auto created, to create such things as novelty URLs. If do create a custom Slug please remember it should not contain any capital letters or special characters other than a hyphen -.
Note: If the slug is edited for a page or department, after it has been linked to, any links will be broken and will need to be redone. If you do not know if the department or page has been linked to, you might want to create a redirect to the new URL.