an instance in GlassFish Server has its own Java EE configuration, Java EE resources, application deployment areas, and server configuration settings
GlassFish Server creates one server instance, called server at the time of installation. You can delete the server instance and create a new instance with a different name.
For many users, one server instance meets their needs. However, depending upon your environment, you might want to create additional server instances. For example, in a development environment you can use different server instances to test different GlassFish Server configurations, or to compare and test different application deployments. Because you can easily add or delete a server instance, you can use them to create temporary "sandbox" areas to experiment with while developing.