while working on osCommerce-3 i got the table structure for category & categories_description as

CREATE TABLE IF NOT EXISTS `osc_categories` (
  `categories_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `categories_image` varchar(255) DEFAULT NULL,
  `parent_id` int(10) unsigned DEFAULT NULL,
  `sort_order` int(11) DEFAULT NULL,
  `date_added` datetime DEFAULT NULL,
  `last_modified` datetime DEFAULT NULL,
  PRIMARY KEY (`categories_id`),
  KEY `idx_categories_parent_id` (`parent_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=5 ; 

CREATE TABLE IF NOT EXISTS `osc_categories_description` (
  `categories_id` int(10) unsigned NOT NULL,
  `language_id` int(10) unsigned NOT NULL,
  `categories_name` varchar(255) NOT NULL,
  PRIMARY KEY (`categories_id`,`language_id`),
  KEY `idx_categories_desc_categories_id` (`categories_id`),
  KEY `idx_categories_desc_language_id` (`language_id`),
  KEY `idx_categories_desc_categories_name` (`categories_name`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

here i am not getting the meanning of indexing

"idx_categories_desc_categories_id",

"idx_categories_desc_language_id",

"idx_categories_desc_categories_name"

What is the use of this indexing.What does it mean?