Web development has two parts: frontend and backend development. Every web or mobile application includes these two parts.
Frontend includes the visual parts of the application. This is what the user interacts with on the screen.
The backend of an application handles the business logic. How the site functions, how the data flows, and how data is stored is all the domain of the backend.
When you combine the two, you get full-stack web development. A full-stack developer is proficient in both front and backend technologies. This also includes the related libraries, frameworks, databases, and server environments.