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 do not see directly. This part of development ensures that data is processed and sent to the front-end, which is what users interact with. Back-end developers work with databases, such as MySQL or MongoDB, to store and retrieve information. They also manage server environments and APIs (Application Programming Interfaces) that allow different software systems to communicate. Overall, back-end development is crucial for creating a seamless user experience on websites and applications.