Environment Variables
Secret passwords your app needs but shouldn't share publicly.
What is Environment Variables?
Your app needs secret keys to connect to services like Stripe or Supabase. You store these in environment variables — a special file called .env that never gets shared publicly. When you see NEXT_PUBLIC_ or VITE_ — those are environment variables.
The most common deployment mistake: forgetting to add your environment variables in your hosting platform. They work locally in your .env file, but the server doesn't have that file.
In Vibe Coding
The #1 reason vibe coded apps break after deployment is missing environment variables. You set them in .env locally, but you also need to add them in your hosting platform — Vercel, Railway, or Cloudflare.
Example
For example: Your app works perfectly on localhost but shows a blank screen on Vercel. You check and realize you forgot to add your NEXT_PUBLIC_SUPABASE_URL to Vercel's environment variables.