JSS, or JavaScript Style Sheets, is a method for styling web applications using the JavaScript programming language. It allows developers to create dynamic styles that can change based on user interactions or other conditions, enhancing the user experience. JSS is often used in modern web frameworks like React to manage styles in a more modular and maintainable way. By using JSS, developers can define styles as JavaScript objects, which can be easily manipulated and reused throughout an application. This approach promotes better organization of code and can lead to improved performance, as styles can be generated on-the-fly rather than being loaded from static CSS files.