Codeigniter is a php framework that makes development of websites easier and faster

Formal description:

CodeIgniter - A popular open source web application framework used with PHP, for the development of full-fledged web-based systems and applications. Based on a Model-View-Controller development pattern, Codelgniter is distinguished by its simplicity. 

In other words:

CodeIgniter - pre-designed structure that simplifies the development of web resources by providing the framework for needed tasks, as well as a simple interface. Thus, avoiding the task of re-writing the code needed to perform these functions.

What is it used for?

CodeIgniter contains many useful features for processing data, images, and all other materials used on your site. Using the standard CodeIgniter, you can increase the level of protection by cutting off most possibilities of hacking. You can also produce a more comprehensive search for information, while maintaining the correct architecture of the resource.