The hidden part of an app that processes requests, applies business rules and handles the database and payments.
The backend is the kitchen behind the counter. When a customer taps place order, the server checks stock, calculates price and tax, talks to the payment gateway, saves the order to the database and sends the confirmation. It is usually written in languages such as TypeScript or Python.
The backend is where real security and business rules live. Prices, permissions and payment verification must be computed here, because the frontend can be tampered with. Its performance also determines how many users your app can handle at the same time. Practical tip: never trust values coming from the browser, such as a product price or discount rate; always recalculate them on the server. Keep secret keys for external services on the server only, never in frontend code.