CSS stands for Cascading Style Sheets, it is a language developed to control the presentation of HTML elements/documents.
HTML has evolved into a language over time that uses elements to define document structure and identify areas of a site and not how those elements should be presented or styled.
Cascading Style Sheets are a series of formatting rules normally found in an external file e.g. as per our templates. Although there is the ability to add CSS to the HTML page itself - this should be avoided where possible.
To see examples of css check out one of our template css files or view the hundreds of tutorials on the web - its a simple language to learn but very important for a web developer to understand