It is utility-first CSS framework for rapidly building custom designs.
Tailwind CSS is a highly customizable, low-level CSS framework that gives you all of the building blocks you need to build bespoke designs without any annoying opinionated styles you have to fight to override.
Every Tailwind utility also comes with responsive variants, making it extremely easy to build responsive interfaces without resorting to custom CSS.
Tailwind uses an intuitive screen: prefix that makes it easy to notice responsive classes in your markup while keeping the original class name recognizable and intact.
While you can do a lot with just utility classes, as a project grows it can be useful to codify common patterns into higher level abstractions.
Tailwind provides tools for extracting component classes from repeated utility patterns, making it easy to update multiple instances of a component from one place.
If it makes sense to be customizable, Tailwind lets you customize it. This includes colors, border sizes, font weights, spacing utilities, breakpoints, shadows, and tons more.
Tailwind is written in PostCSS and configured in JavaScript, which means you have the full power of a real programming language at your fingertips.
Tailwind is more than a CSS framework, it's an engine for creating design systems.
Check out more on the official Tailwind CSS Website.
Also, you can check out the official Tailwind CSS Tutorials.