Tailwind CSS is a utility-first CSS framework made by Tailwind Labs. CSS is the language that sets the layout, colors and fonts of a web page; a utility class is a class name that sets one small style property, such as flex, pt-4 (top padding) or text-center.
A developer combines many of these classes directly in the HTML markup instead of writing a separate style sheet. Prefixes apply a class only in some conditions: a screen-size prefix such as md: applies it from a given breakpoint, and dark: applies it in dark mode. The theme (fonts, colors, sizes) is set with CSS variables. When the project is built for production, Tailwind scans the source files and keeps only the CSS for the classes that are used, so the final style sheet stays small; the site states that most projects send less than 10 kB of CSS to the browser.
Tailwind CSS is open source under the MIT License and free to use. Its code is on GitHub.