This document provides an introduction to CodeIgniter, including essential skills needed to use it, an overview of its model-view-controller (MVC) pattern, and flow charts illustrating how it works. The flow chart shows that CodeIgniter uses index.php as a front controller to initialize resources, routes HTTP requests to controllers, applies security filtering, loads models and libraries, and renders the final view to send to the browser. The MVC pattern sections explain that models represent data structures, views present information to users, and controllers serve as intermediaries between models, views, and other resources needed to process requests.