IRC stands for Internet Relay Chat.  The IRC sytem provides a way for Drupal users to communicate in real time.

To begin, you must download an IRC client, for example XChat.  Then configure the client to use the freenode network, irc.freenode.net.  Popular Drupal channels include:
 

#drupal   Coding questions and other general questions.

#drupal-support  Support for any non-coding issues.

#drupal-themes  Support for theming.

For more information see the Drupal site: http://drupal.org/irc.