Material-UI is a popular React component library that implements Google's Material Design guidelines. It provides a set of pre-designed components, such as buttons, forms, and navigation bars, which help developers create visually appealing and user-friendly web applications. By using Material-UI, developers can save time and effort, as they don't need to design components from scratch. The library is highly customizable, allowing developers to adjust styles and themes to fit their specific needs. It also includes features like responsive design and accessibility support, making it easier to build applications that work well on various devices and for all users.