what is a node

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}/`);
});Is a child of EventTarget
Node = DOM objects =  Attr, CharacterData (which Text, Comment, and CDATASection are all based on), ProcessingInstruction, DocumentType, Notation, Entity, and EntityReference.
function isNode(o){
  return (
    typeof Node === "object" ? o instanceof Node : 
    o && typeof o === "object" && typeof o.nodeType === "number" && typeof o.nodeName==="string"
  );
}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/