A UI Library is a collection of pre-designed user interface components that developers can use to build applications more efficiently. These components can include buttons, forms, icons, and navigation elements, which help ensure a consistent look and feel across different parts of an application. By using a UI library, developers can save time and reduce errors, as they don't have to create these elements from scratch. Many popular UI libraries, such as Bootstrap and Material-UI, provide ready-to-use components that are customizable and responsive. This means they can adapt to different screen sizes and devices, enhancing the user experience. UI libraries also often come with documentation and examples, making it easier for developers to implement them in their projects.