What is CSS ?
CSS = Cascading Style Sheets
In short, with CSS, the website will be more pleasing to the eye.
The way a website look to the user can be controller by CSS, such as text alignment, the color, how big or small the text is, and many more. CSS also support more complex operation out of the box.
CSS is optional for HTML, but HTML is not optional to CSS, because without HTML, CSS is just bunch of code.
Fun fact: HTML initial release is 1993, and CSS initial release is 1996. Just imagine (or googling) how website looks without CSS (before 1996). And back then, HTML
<table /> tag still very popular for designing website layout.
Sample of CSS code :
Now the color of h2 tags in HTML document becoming green.
What is Minification (from Minifier) ?
Minification is the process of removing all unnecessary characters from the source codes without changing their functionality or behaviour. So, the size of your code will be reduced without changing the result of the code.
Minification can remove few things from source code such as spaces and newlines. Or even can change the structure of the code while maintaining the same result (this example is quite extreme, but possible. The process can be quite the same as in the compiler).
The drawback of minification of course the code can be harder to read, thats why minify can be called uglify.
If you found problem with your css try to not check the Enables merging of @media rules with the same media query