Node
Node is a program that lets JavaScript run on your own computer or a server, not only in a browser.
In a simple example
To build your site you install Node once, and your tools use it to run the build.
Why it matters to you
Many AI coding tools ask for it first, so you will meet it early.
Do not confuse with
Node is not a website and not a language. It is the engine that runs JavaScript outside the browser.