CSS (short for Cascading Style Sheets) are a part of the HTML standard that tells a browser how to display HTML elements. There are a number of benefits for using CSS to design your web pages. An external style sheet can make pages easier to design, manage and update, because every page can reference the same CSS file for style guidelines including font, color, size and more. Pages also can load quicker and can be easier for search engines to index because you reduce the amount of code that is needed for each page. Furthermore, you move important content to the top of your html file.

The other important aspect of CSS as it relates to SEO is that it allows you to use standard tags like <h> headline, or <ul> unordered list and format them using CSS. Search engines can use these tags recognize important content or crawl your site more completely.

If you are interested in learning CSS, W3 schools has a nice intro tutorial to CSS.