CSS, or Cascading Style Sheet, is a way to cut down considerably on all the code entered in a website. For example, this site has been written with the text set to be Arial, then if Arial is not available, Helvetica (and if that's not available then your browser's default font). This command has been inserted into each and every tag that contains text! All the text (apart from examples of alternative layouts) is justified, which requires the justify command for each paragraph.

CSS avoids the need for all this by having one single sheet with commands which are referenced from each web page. I do not intend to go into the inner workings of CSS here as it is a whole topic of its own, but suffice to say it
A) saves a lot of typing
B) enables a common command throughout the site to be amended on just one page. Just imagine the amount of work involved if going through a 1,000 page site to amend a font size compared with just one amendment in the style sheet!

My last word here on style sheets - a site may have several, or many, different ones allowing different sections to display in their own way.