This is a CSS Framework designed as a so called utility-first framework. This means that this framework does not provide you any already built component like Bootstrap or Foundation, instead it provides you with css classes that resemble to css properties to build your components.

It is the most advanced utility-first framework out there and although it has been around for many years, it has lately increased it's popularity with more and more use cases. They also offer already built-in components made with Tailwind CSS.

The documentation is extensive and very well presented.