CSS stands for Cascading Style Sheets, which is the code mechanism designers use to control the layout of a website.
CSS and HTML work together, the HTML determining the contents of a page and the CSS determining what the page looks like. CSS code is held in stylesheets externally, but can also be added directly to an HTML page (inline CSS) . A stylesheet can control the layout of multiple pages at once saving a lot of time embedding style details to HTML code on a page-by-page basis.
Find out more about CSS at W3 Schools.