what is node js

Javascript was always a client side language until node.js.
Common server side languages include PHP, Python, Perl, Ruby
and several more. Node enables you to use Javascript server side.
This now means you can have a consistent language both ends
which could not be done prior to Node.

npm install http then write a webserver...
const http = require('http');
const PORT = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World');
});

server.listen(port, () => {
  console.log(`Server running at PORT:${port}/`);
});

Node js allows you to run javascript outside of your browser
ex: you are able to run it on your terminal

/*What is Node.js?
Node.js is an open source server environment
Node.js is free
Node.js runs on various platforms (Windows, Linux, Unix, Mac OS X, etc.)
Node.js uses JavaScript on the server*/

Javascript generally runs on Browser(Client).
An  Example Browser - Chrome, has v8 Engine, which compiles Javascript to a code the computer can understand.

Thus Node.js can run Javascript on Server with Chrome's V8 Engine.

Further Reading: https://www.freecodecamp.org/news/what-exactly-is-node-js-ae36e97449f5/