How this maps to your app

Now put the whole picture over a vibe-coded app, because every piece you'll build lives on one side of the dance:

• The frontend is everything that runs in the browser — the buttons, forms, and layout the user sees and clicks. It's the client. It mostly sends requests and displays responses.
• The backend is your code running on a server — the logic that decides what to do with a request, talks to the database, and builds the response. It's the server.

A real interaction stitches it all together. A user clicks "Sign up" (frontend). The browser sends a POST request over HTTPS to a path like /api/signup, carrying the form data. Your backend receives it, creates the account, sets a cookie so the user stays logged in, and sends back a 200. The frontend sees the success and shows a welcome screen. Every word in bold there is something you just learned — and that's the entire shape of nearly everything you'll build.

You don't have to hold all of this in your head at once. But when a later chapter says "the request hit the server but came back 500," or "set this header," or "this route handles the POST," you'll now know exactly which part of the dance it's pointing at. That mental model is the real prerequisite — and you've got it.