CSS is the language commonly used with HTML to add more stylistic elements to a webpage, like navigation menus, borders, and margins. It can also change things like the color but that can also be done with HTML.

CSS is formatted so that in between the starting and ending heading tags in a HTML document, add the style tags(<style> </style>) and in that style tag put the inside of the tag you want to add CSS to so if you want to add CSS to <h2> tag  you just put h2 then braces followed by what you want to edit like border, then the specifics like the size , color and type of border.

A clearer example of CSS is shown in the picture below: