While how a website looks and interacts on the surface involves design and front-end development, back-end development includes all the programming and systems that power everything going on below the surface. It is what makes websites work. Even the simplest website has back-end functions required to make things show up properly and allows interaction by users. Databases, APIs, functional code and more all come under this heading.
ProDevs back-end developers know the ins and outs of every popular scripting language or method to deliver the best possible outcome for website users. After all, most sites have interactive components that require a lot of connections and activity behind the scenes. User management, shopping carts, and content management systems like WordPress all use BE development.