A render array is a fundamental building block of the Drupal Content Management System. It is an associative array of data structures containing the content of a web page. The array contains the data itself, as well as hints about how it should be "rendered."
When a web page is served, the data is formatted for display, using the associated hints, according to a site-wide or page-specific theme. This separation of what the data is, and how it is presented to the user, allows the content manager enormous flexibility in modifying the presentation of data in a uniformly global manner.
The following is an example of a simple Drupal render array:
<?php $page = array( '#show_messages' => TRUE, '#theme' => 'page', '#type' => 'page', 'content' => array( 'system_main' => array(...), 'another_block' => array(...), '#sorted' => TRUE, ), 'sidebar_first' => array( ... ), 'footer' => array( ... ), ... ); ?>