A ReactJS developer is a professional specializing in developing applications using the React library, a popular JavaScript framework for building user interfaces. React development involves creating reusable components, managing application state, and handling data flow between components. React developers must be proficient in JavaScript, HTML, CSS, and related technologies and have experience with version control, testing frameworks, and modern development tools. They must also understand software design patterns, algorithms, and data structures to write efficient and maintainable code. Overall, React development is a crucial skill for building modern web applications that are fast, responsive, and scalable.