JavaScript frameworks are pre-written libraries of code that help developers build web applications more efficiently. They provide a structured way to organize and manage code, making it easier to create interactive features and user interfaces. Popular frameworks like React, Angular, and Vue.js offer tools and components that streamline the development process, allowing developers to focus on building functionality rather than starting from scratch. These frameworks often come with built-in features such as routing, state management, and templating, which simplify common tasks. By using a framework, developers can ensure their applications are more maintainable and scalable, ultimately improving the user experience on websites and applications.