IP address and port

An IP address identifies a computer on a network, like 203.0.113.10. A port number picks out one program on that computer, because many can be listening at once. Together they say exactly where a message goes.

The errors an agent hits while running your app are often about this and nothing else. "Address already in use" means two programs want one port. "Connection refused" means nothing is listening there. Neither is a bug in the code you just changed.