What happens when a server object is sent to a listener?

1) What happens when one server listens to another?

var io = require('socket.io'); var http = require('http'); var server = http.createServer(); var io = io.listen(server); 

2) And when is a port already listening to any transmitted?

 var express = require('express'); var app = express(); var server = app.listen(3033); var io = require('socket.io').listen(server); 

It is also interested in the context of sockets.