What is doctrine and how easy to learn it for beginners?

Doctrine is an ORM (Object Relationnel Manager) that allows you to manage database as php objects

here is a  documentation of Doctrine

integration with codeigniter framework  codeigniter-doctrine

Doctrine is an Object Relation Mapping software for PHP it's very similar to Eloquent (used by laravel).It implements the Active Record Architectural Pattern which has to represent every Database Table as Class in your code and evey record in your database table as Object of that class.In other word you will no more need to write queries to access the database instead you can create objects of the tabes in your code and change their properties to affect the database rows.