They are Block--Element__Modifiers (BEM). It's a style of css meant for developer readability and code reuse.

In this instance you have the following:

.btn { //the base code for a button }

.btn--s { font-size: 12px; }

.btn--m { font-size: 14px; }

.btn--l { font-size: 20px; border-radius: .25em!important; }

Now if I want to make two buttons, one with small font, the other with large font, I can simply do this:

<button class="btn btn--s"></button> <button class="btn btn--l"></button>

Personally, I like writing with BEM methodology rather than repeating style rules over multiple classes.

> Centered, full-width on mobile, with a max-width of 16em on larger screens.