A slug as the part of a URL that identifies the relative URL part of the page in humanly understandable keywords. It is said to be an end part of the URL. which is similar to the basename of the URLs structure. It is also called as a short label or short name of the URL or basename of the URL.
The slug may contain in lowercase letters, uppercase letters, numbers, dash(hyphen-), underscore(_) but lowercase letters and good keywords are best for SEO.