Cascading Style Sheets.
Traditionally HTML was formatted using attributes within the tags. They looked like this:
<p color=red>This paragraph would have red text</p>
If you wanted a paragraph to have red text you just added color=red to the opening paragraph tag. That was simple but limited and flawed so CSS was invented. The old methods still work on most browsers but don't use them as they are embarrassing. If you ever see HTML with this sort of thing in throw it away as it is either ancient or written by someone who is ignorant.
CSS allows you to change the look and layout of Web pages. You create the page with HTML and then style it with CSS. CSS is very powerful so can take some learning. It can change colour, size, alignment and even make some items invisible or hidden behind others (which will make sense eventually!).
On this site you will learn HTML first (so that you can put stuff into a Web page). Once you can do that you will meet CSS and that is much more fun.