How to understand socket.io initialization?

Question

Socket.io

It needs to be integrated into a ready-made application based on express generator. But for this, you first need to figure out how socket.io is initialized.

var express = require('express');
var app = express();
var http = require('http').Server(app);
var io = require('socket.io')(http);

What is done in the http variable? Why is .Server(app) needed?