What is CGI?
"CGI" stands for "Common Gateway Interface." CGI is the method by which a web server can obtain data from (or send data to) databases, documents, and other programs, and present that data to viewers via the web. More simply, CGI is programming for the web. A CGI can be written in any programming language, but Perl is the most popular, and for the course of this book, Perl is the language we'll be using.

Why learn CGI?
If you're going to create web pages, then at some point you'll want to add a counter, a form to let visitors send you mail or place an order, or something similar. CGI enables you to do that and much more. From mail-forms and counter programs, to the most complex database scripts that generate entire websites on-the-fly, CGI programs deliver a broad spectrum of content on the web today. If you've ever looked at a site such as Amazon.com, DejaNews, or Yahoo, and wondered how they did it... the answer is CGI. CGI experience is also in high demand from employers now; you could substantially improve your salary and job prospects by learning CGI.
From: CGI 101, learn CGI usnig CGI101 tutorial