CodeIgniter is an application development framework, which can be used to develop websites, online based software using PHP. It is an Open Source framework. It has a very rich set of functionality, which will increase the speed of website development work.
If you know PHP well, then CodeIgniter will make your task easier. It has a very rich set of libraries and helpers. By using CodeIgniter, you will save a lot of time of your life, if you are developing a website or software from scratch. Not only that, a website/software built in CodeIgniter is secure too, as it has the ability to prevent various attacks that take place through websites or software.