First and foremost you are going to have to get it into your head that CGI is a concept, not a thing. (If you need to understand what I mean by that, use a dictionary.)

That said, this is not an appropriate place to ask what cgi means. Discussion is widely available on the web.

If you have been doing a lot of reading on this and still do not get it, you need to ask someone in person. Asking this here is akin to trolling. (You may as well ask a journalist what the letter r means.)

If you really still don't get it, maybe this is not your line of work.