Back end web development is all responsible for calculations, storing information, interactions with database and performance. Backend is a part of your website that a visitor and customer could never see. It includes work with a server, an application and a database.
Say when you book a movie ticket or sign up a newsletter, you interact with the front end, but when the information is entered it goes to a database and get stored. Without a backend even the most beautifully crafted website meets with failure.