Back-end development, or server-side scripting, deals with the behind-the-scenes tasks. These developers are tasked with storing and organizing data and making sure that all of the client-side features run correctly and smoothly.
When the end user interacts with the client-side features, they send requests to the server-side. The server-side then "responds" by providing the information that enables the front-end code to perform its function in the end user's web browser.