CodeIgniter is an open source PHP framework for building web applications. According to some sources, such as W3techs, PHP is the most widely used programming language in the world.
What is a framework?
A framework is a software environment in which you build an application. It allows you to develop projects much faster than if you would build them from scratch. It has a rich set of libraries for often used tasks and a clear interface.
What makes CodeIgniter unique?
CodeIgniter is not the only PHP framework, there are many others like CakePHP. However, CodeIgniter has a number of advantages:
It is lighter than other frameworks, but very powerful.
In short, CodeIgniter is a framework for building very fast web applications.