Directory Listing is a webpage that shows the files and directories on a web server that make up the website. Every website has a directory that hosts the files; however, listing the directories publicly so that they are viewable by anyone is a security risk.
If Directory Listing is enabled on a website, it means the web server that hosts the site is configured to automatically show the directories. Hackers can then check to see if the directories are visible and view the paths of the files which aid in their ability to hack the site.