....Can briefly explain what CSS is?....Well, I'm sorry, I just started to understand Twine...
CSS allows you to place code objects and change their appearance. That's it in short. To understand what it is, watch various videos about HTML and CSS. They always work together, and Twine is also tied to them. There is also JS, but that's after mastering the first two. In general, these three can be called the basis of website building and layout.