It is a PHP framework.

When we write a program with PHP, we can write:

• in vanilla PHP (it means using the language as it is)
• using PHP framework (it means a set of commands that summarize the PHP functionality)
• using a PHP CMS like WP, Joomla, Drupal e.t.c ( it means using prebuild blocks of code, or even first write the blocks)

If we write in PHP, we must have libraries written to speed the procedure, but in general, it is more difficult and time-consuming.

What are the prerequisites in order to write in CI

• you must know PHP and especially OOP
• you must know MVC model technology and a good reason to learn CI is to understand this model 

If you are one or two people in the group it is good to know Frond-end languages/frameworks like:

• HTML
• CSS
• JQuery
• JS
• Bootstrap/Foundation frameworks e.t.c.

Other topics that may be needed:

And Database Technology like MYSQL

• Commands for CLI (UNIX/Linux  like commands)
• FTP/SFTP