CSS is the ‘code’ that makes your site look how it looks.
It is basically the ‘style’ of your site. CSS should be optimised to use as little code as possible to make your site load quickly. Poorly written CSS will inform the search engines that the build quality of your site is sub-par, which can, in turn, affect your rankings.
The style sheets are also read from bottom to top so the most important code is usually placed towards the bottom of the sheet by developers, along with the responsive code.