GlassFish is an open source application server that provides a Java EE platform for developing and deploying Java and Ruby on Rails applications. It offers features like clustering, load balancing, and monitoring that benefit Rails applications. Ruby on Rails applications can be easily deployed to GlassFish through directory deployment, a GlassFish Gem, or by creating a WAR file. NetBeans IDE provides convenient development and debugging of Rails applications deployed to GlassFish.