The behind-the-counter part of your app that handles data, accounts, and rules.
Like the kitchen behind a restaurant's counter, the backend is the part of your app that does the work out of sight.
It stores data, checks rules, talks to other services, and sends results back. In this course, Supabase stores data and exposes an API, and Modal runs server jobs.
In practice, backend code runs on a server, a computer on the internet, not on the phone. It might send emails with Resend or charge cards with Stripe.