This document introduces CSS (Cascading Style Sheets) and how it is used to style HTML documents. CSS code belongs in a separate file than HTML and is linked using the <link> tag. CSS allows you to control things like backgrounds, colors, fonts and layout. An example is provided of how to change the font color of an <h1> tag by selecting it with CSS and specifying the new color value. The key aspects of CSS code, like selecting elements, properties, and values, are explained.