CSS Class Brackets?

I’ve been away from the web for almost 5 years, so I apologize for asking this simple question.

As I was looking at the Twenty Sixteen WordPress theme, I noticed that there is a class that has brackets. Since I’ve been gone, they came out with CSS3. So things have changed. I provided a snippet below to show an example of what I mean. There’s parentheses and brackets in the classes now. What do they mean?

audio:not([controls]) {
	display: none;
	height: 0;
}

[hidden],
template {
	display: none;
}