CSS stands for Cascading Style Sheets. CSS allows you to style the HTML elements that make up your website and their content. CSS can be implemented through a seperate file which is then accessed through the html file. This allows you to create just one stylesheet which can be used on all the webpages within your website. For example a header tag (h1, h2, h3) can be styled throughout your website even when you only style it once on one stylesheet. If you want to style an element seperately you can identify it using an id. An id is a unique identifer, meaning a given ID should only be used once on a webpage and can the be referenced in your stylesheet. To apply a style to more than one element at a time, a 'class' can be used. Defining the styles of a class, means you can add the class to many elements throughout your webpage and they will all be styled accordingly. CSS is very powerful and really allows you to give your website a unique finish.
I have been building websites since I was about 11 years old and CSS has always been a focus of mine. It's great to experiment with and I find that while there are limits to what you can do, it is very powerful. Simple styling, as in the primary layout of your page for example, would be impossible without CSS. But it has so much more to offer. Now we are in the era of responsible responsive styling. When designing, there are two ways you can plan your website in regards to its styling aspect. The first is mobile first - where you design first for the mobile layout and proceed from there to sculpt for larger screen sizes as the size increases. The second way is the opposite - to design for all screen sizes. The second is rather difficult in most regards, but with practise it is acheivable. The idea is to not really design for a specific size but to have the design complement all screen sizes. There is obviously a minimum size a screen can be (for now) but there is no limit (in a way) to the maximum size. A mixture of these two principles helps me to design for my clients and CSS plays a massive role in the design. Utilizing CSS queries for screen sizes allows me to change the layout on the screen for different sizes or breakpoints. Always experiment is what I say, there's always more than one way to do something with CSS.