Drupal is a powerful, flexible platform for building applications, but not something that handles realtime notifications easily. Node.js is a breath of fresh air in the Open Source web server landscape. It makes writing applications that handle thousands of open connections at the same time easily.
The Nodejs module integrates Drupal with Node.js, allowing for the best of both worlds. Realtime chat, push notifications and help desk functionality can all be easily added to your Drupal site via the Nodejs module, without the usual scalability and performance issues associated with these technologies on the LAMP stack.