Back-end development involves building and maintaining the server-side logic and database of a website or web application. It includes tasks such as writing code in languages like Python, Ruby, Java, or Node.js, designing and implementing APIs, managing databases, and ensuring the security and scalability of the application's infrastructure. Back-end developers work behind the scenes to handle data processing, user authentication, and server communication, enabling the functionality and dynamic content of the website or application to operate smoothly.