React JS is a powerful JavaScript library developed by Facebook. It's used for building dynamic and interactive user interfaces, especially for single-page applications where you want a seamless user experience without the whole page reloading. With React, you can create reusable components, which means you can break down the UI into small, manageable pieces and reuse them across your application. It uses a virtual DOM to efficiently update and render just the components that change, making your app faster. Think of React JS as a tool that helps you build more responsive and interactive web applications with less code.