The "back-end" refers to the part of a software application or website that users do not see. It is responsible for managing the server, database, and application logic. This includes processing requests, storing data, and ensuring that everything runs smoothly behind the scenes. Back-end development typically involves programming languages like Python, Java, or Ruby, and frameworks such as Node.js or Django. Developers work with databases like MySQL or MongoDB to store and retrieve data efficiently. Overall, the back-end is crucial for the functionality and performance of any digital platform.