Start here
A route is the server code for one endpoint. It says: when this method and this path arrive, do this.
app.get('/hello', (req, res) => {
res.json({ message: 'Hello from the server' })
})
Read it as: when a GET request for /hello arrives, reply with this JSON. That's the shape of Express, the tool most Node.js servers use. You'll write it here on a practice server, with nothing to install.