The 'backend' is the brain behind a web application. It's the invisible part that handles data, processing and system logic. Backend developers create and maintain servers, databases, and server-side applications that power the entire website. They work with programming languages like Python, Ruby, PHP, and JavaScript to create APIs and web services that provide data to the frontend. Without a robust backend, the frontend wouldn't function. Backend developers play a crucial role in ensuring that an application is secure, fast, and efficient. It's a growing field with an increasing demand for skilled professionals.