The answer is both. React is a library for building user interfaces and is used for both front-end and backend development. It is used to create the view layer of an application, or the part of the application that the user interacts with. This includes creating user interface elements such as buttons, forms, and other components that the user interacts with. In addition, React is used to create the logic behind the user interface, or the code that is responsible for making the user interface work.
React is also used for backend development. It is used to create the server-side code that is responsible for the logic behind the application, as well as the API endpoints that enable the front-end code to communicate with the backend. Furthermore, it is used to create database queries that are used to store and retrieve data from the database.