Rails UI is an extensive toolkit designed to accelerate the development of Ruby on Rails applications. It offers a collection of professionally-designed templates and reusable components that seamlessly integrate with the Ruby on Rails framework, allowing developers to focus on building impactful features rather than spending time on UI design and implementation. Its key features include a wide range of pre-built components, such as accordions, alerts, badges, buttons, and more, which can be easily customised and integrated into your Rails projects.