CGI stands for common gateway interface. When you log into file manager in cPanel for the first time, you'll notice a folder in your Public_html folder called your "cgi-bin". CGI in the most basic of terms is a way that the webserver communicated information to a CGI program. The cgi-bin folder is where you will store the scripts such as Perl (.pl) that your website will use.
If your website contains a form for your visitors to fill out, it's likely this form will be handled with a script placed in the cgi-bin folder. If you would like more information on creating a form on your website please see our link: Adding Contact Forms to your Website