What is it called when a class connects to a selector in external <style> doc?

I am looking at tutorials for CSS. Sometimes they bring something new in the example that hasn’t been explained yet. An example is when a class is hitched to a selector.

div {
  background-color: green;
}

div.first {
  opacity: 0.1;
}

div.second {
  opacity: 0.3;
}

div.third {
  opacity: 0.6;
}
</style>
</head>

What is

div.first {
}

called? I don’t know what div.first is called when written like this and i want to look it up or have it explained to me.

Thanks a lot.

div.first selects all div elements whose class is first

there is a good reference for selectors here