Node.js
also Node
A runtime that lets JavaScript run on a server instead of only in the browser, powering the backend of most JS apps.
Analogy
Like the kitchen where the food is actually cooked, out of the customer's sight.
Why it matters
It's what runs your backend logic and API routes, and Next.js itself runs on Node, so it's core to full-stack builds.
In practice
A Node server receives a webhook, processes it, and writes the result to a database.