Tailwind CSS is a utility-first CSS framework that allows for rapidly building custom designs using utility classes rather than pre-designed components. It provides low-level utility classes that can be used to style self-created components and build completely custom designs directly in HTML without needing to leave the file. The document then outlines how to install Tailwind CSS, its workflow, examples of how to build common components like alerts and buttons, and advantages like being responsive, customizable and friendly for building components.