simply saying CSS is the tihng that makes your life easier by preventing you coding the same & same properties of your site over and over again
ex: you need all the paragraphs of your site to have the first letter bold
you obvously cannot put the code for it over and over again
but you can just make a single CSS, put al other tihngs you need with it
then attach it to everysingle page
so just in case you decide that you dont need the first letter bold, you simply need to edit one single thing