As described perfectly on its home page, Express is a minimal and flexible Node.js web application framework, providing a robust set of features for building single, multi-page, and hybrid web applications. In other words, it provides all the tools and basic building blocks you need to get a web server up and running by writing very little code. It puts the power to focus on writing your app and not worry about the nuts and bolts that go into making the basic stuff work in your hands.

The Express framework is one of the most popular Node-based web frameworks and one of the single most popular packages available in npm. It is built based on the Sinatra web framework, which is quite popular in the Ruby world. There are a lot of frameworks across languages that take inspiration from Sinatra's simplicity such as PHP's Laravel framework. So Express is the Sinatra-based web framework in the Node.js world.

If you look at a sample piece of code, one of the most basic implementations of Express, you can see how easy it is to get a web server up and running, for example:

var express = require('express');
var app = express();
app.get('/', function(req, res){
   res.send('Hello World');
});
app.listen(3300);

The beauty of Express is that it makes building and maintaining the server code for a website simple.