CSS stands for Cascading Style Sheet. Most commonly referred to CSS, it is used to style a webpage. For example, you can modify the stylesheet for your WP theme to change the fonts, colors, margins, lines, height, width, background images, advanced positions and many other things.
Warning: If you want to modify the theme’s stylesheet, you should always create a child theme. Never make changes to the stylesheet in the original wordpress theme. Why you ask? Well, if your theme needs to be updated, all the changes you’ve made to the stylesheet will be overwritten by the update.