What is Express.js and how do I create a basic app?

<!-- BEGIN COPY / PASTE -->
        const express = require('express');
        const app = express();
        const port = 3000;

        app.get('/', (req, res) => {
          res.send('Hello World!');
        });

        app.listen(port, () => {
          console.log(`Example app listening at http://localhost:${port}`);
        });
        <!-- END COPY / PASTE -->