A Cascading Style Sheet (CSS) can be what applies the style for a webpage written in HTML.  When we build a website, we have a specific design/look in mind.  A CSS allows us to apply that look sitewide, keeping a consistent feel throughout the entire site - but this way you don't have to try to figure out what color is used here or there, or what font this page is, etc.  The CSS applies all of those details for you.