Css stand for Cascading Style Sheets.
basically css helps you design your site, it allows you to color, customize, size, resize, and most importantly position various elements in your HTML code.
a simple example would be coloring the background of a web page
<style type="text/css">
background-color:blue;
</style>
this is inserted into the header of a web page to change the background to blue.
Alternatively, you can call css already written into an external file(this is useful for making site themes.)