CSS stands for Cascading Style Sheet. It a programming language used to describe presentation of the HTML Web pages.
Using CSS you can change background color, font color, padding, border and much more on your web page.
CSS is designed to help enable the separation of presentation/design elements and content, including layout, colors, and fonts. This separation helps improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple web pages to share formatting by specifying the relevant CSS in a separate .css file, and reduce complexity and repetition in the structural content. Easier for the web developers to create websites.