Step 27 Learn Typography by Building a Nutrition Label

In the HTML we put class with value “divider lg” but why in CSS we put “.lg” and “.md” and what is “.md”, where it comes from? I hope someone could explain these to me. Peace.

 <div class="divider lg"></div>
.lg {
  height: 10px;
}
.lg, .md {
  background-color: black;
}

the md(medium) class will be utilized in step 37 for the thinner bars of the nutrition label.