The "Back-End" refers to the server-side of a web application or website, where all the behind-the-scenes processes occur. It includes the server, database, and application logic that manage data and ensure everything runs smoothly. Developers use programming languages like Python, Java, or Ruby to create the back-end, which communicates with the front-end to deliver content to users. In simpler terms, while the front-end is what users see and interact with, the back-end is responsible for storing and processing data. It handles tasks such as user authentication, data storage in a database, and server responses, making it essential for a functional web experience.