The back end is the part of an application that runs on servers, invisible to the user: it receives requests from the interface, applies business rules, reads from and writes to databases and sends back results. It is the counterpart of the front end, the part displayed in the browser or mobile app.
When you check out an online cart, the back end checks stock, computes the price, calls the payment service, records the order and triggers the confirmation email.
Back-end development covers API design, data modeling, security, performance and integration with other systems. Common languages are Java, Python, Go, PHP, Ruby, C# or JavaScript and TypeScript through Node.js. The associated frameworks (Spring, Django, Symfony, Laravel, Rails) give this work its structure.
Why it matters when hiring
The language matters less than people think: a good back-end developer moves from one language to another in a few weeks, whereas mastering data modeling, concurrency and scaling takes years. In an interview, ask them to walk through a production incident they solved. The trap: insisting on the exact language instead of the right engineering level.