I never heard of it, nor do I know what it does. Anyone shed some light on this for me?

It's just a place where you can add your own CSS codes to change the way Subeta looks for you personally. So like if you want a different background color or font size or something, you can change it there.

CSS is code that be used to manipulate the way things on Subeta look to you On a large scale you can use it to override the entire layout of Subeta, You can find some free-to-use ones HERE

On a smaller scale you can use it to move tiny things like deleting the facebook buttons or making floating items appear in a certain spot no matter the page you are on

and

Thanks!