Back-End Development refers to the server-side of web applications, where the logic, database interactions, and server configuration occur. It involves programming languages like Python, Java, or Ruby to create the functionality that users don’t see but is essential for the application to work. Back-end developers manage databases, handle user authentication, and ensure that data flows smoothly between the server and the client-side interface. They often work with APIs to connect different services and ensure that the application runs efficiently and securely.