CGI is the part of the Web server that can communicate with other programs running on the server. (Shishir Gundavaram, "CGI Programming on the World Wide Web")
Common Gateway Interface (CGI) is a type of program that can be very simple or very complex, depending on the programmer's skill level and the website needs. CGI allows one program to access another program on the Web and to transmit (or "pass") information to the other program, which the other program processes or responds to. The second program then reverses the direction of information passing, or transmission, and sends the response to the original data back to the first program. CGI is at work in programs like feedback requests of registration information or image maps that allow you to access information for specific locations, like Wyoming or Alabama.
Common Gateway Interface," is a standard way for a Web server to pass a Web user's request to an application program and to receive data back to forward to the user."
In a simpler basis, the CGI is what is used when you open up a Web page or fill out anything in a webpage (such as a password) and the CGI will send the information to an application to read the information you entered and then proceed to bring information back in a webpage (such as logging in or having a screen that says the login failed.