This is a network error that occurs when attempting to connect to a Server. It signifies that no server is listening at the hostname and port you specified. It simply indicates that the machine exists, but that no services are listening on the port supplied.
In most cases, something is blocking a connection to the port or hostname. Either a firewall is preventing the connection or the process hosting the service is not listening on that exact port. This might be because it is not operating at all, or it is listening on a different port.