CSS means “Cascading Style Sheets” and in layman’s terms refers to a rule-set that an html page can draw on to present many pages in a certain way. The result when compiled into a WebSite is called a css “style sheet”, and can be used to influence the design of the entire site, or only sections of the WebSite. In other words, a CSS file may say that all heading text be red and a certain size. Then any pages in the site directed to use the CSS stylesheet will use those rules and will present themselves accordingly.
This means if you need to change the appearance of something (like headers, main body text, image borders, background texture, whatever), across the site, you change the rule in the CSS file instead of having to change each and every page individually. When using a CMS, avoid too much formatting of the text, Stick with basic things like bold and italic, or a heading designation.
Remember, conversely – if using css style sheets site-wide, generally; any individually formatted content ignores CSS rules and would revert back to needing to be changed by hand hard coding if you needed to make any additional future further changes.