The backend is the part of a system invisible to users: the server, database and business logic that process requests, store data and enforce security.
When you click "buy now", the frontend sends a request and the backend does the work: checks stock, calculates the price, authorises payment, records the order, sends the email. The more serious the product, the more happens on this side.
Backends are written in languages like JavaScript/TypeScript (Node.js), PHP or Python, with databases such as PostgreSQL or MySQL. The marks of a good backend are security (data protection, attack resistance), scalability (works with 10 and 10,000 users) and maintainability (clean, documented code).
Warning sign: if a vendor cannot explain how they secure data and what happens when traffic grows – the backend will be the product's weakest link.