Tailwind CSS is a utility-first CSS framework that provides a large set of low-level utility classes to build custom designs without writing CSS. It focuses on rapidly building custom user interfaces by applying predefined classes directly in the HTML.

The tailwind.config.js file is used to customize Tailwinds default configuration, such as defining custom colors, spacing, fonts, and breakpoints. It allows you to extend or modify Tailwinds utility classes according to the specific needs of your project.

Utility-first CSS is a design approach where you use single-purpose classes to style elements. Instead of writing custom CSS, you use predefined utility classes to apply specific styles, like text-center for centered text or mt-4 for margin top of 4 units. This makes it quick to style components without writing custom CSS.