What does addListener do in node.js?

Due to the fact that Node.js works event-driven and executes an event-loop, registering listeners allow you to define callbacks that will be executed every time the event is fired. Thus, it is also a form of async. code structuring.

It's comparable to GUI listener, that fire on user interaction. Like a mouse click, that triggers an execution of code in your GUI app, your listeners in your example will be run as soon as the event happens, i.e. a new client connects to the socket.