Although an understanding of HTTP is not strictly necessary for the development of CGI applications, some appreciation of "what's under the hood" will certainly help you to develop them with more fluency and confidence. As with any field of endeavour, a grasp of the fundamental underlying principles allows you to visualise the structures and processes involved in the CGI transactions between clients and servers - giving you a more comprehensive mental model on which to base your programming.
Underlying the user interface represented by browsers, is the network and the protocols that travel the wires to the servers or "engines" that process requests, and return the various media. The protocol of the web is known as HTTP, for HyperText Transfer Protocol. HTTP is the underlying mechanism on which CGI operates, and it directly determines what you can and cannot send or receive via CGI. Although an understanding of HTTP is not strictly necessary for the development of CGI applications, some appreciation of 'what's under the hood' will certainly help you to develop them with more fluency and confidence.
Tim Berners-Lee implemented the HTTP protocol in 1990-1 at CERN, the European Center for High-Energy Physics in Geneva, Switzerland. HTTP stands at the very core of the World Wide Web. According to the HTTP 1.0 specification,
The Hypertext Transfer Protocol (HTTP) is an application-level protocol with the lightness and speed necessary for distributed, collaborative, hypermedia information systems. It is a generic, stateless, object-oriented protocol which can be used for many tasks, such as name servers and distributed object management systems, through extension of its request methods (commands). A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.