toggleClass: How does this CSS work?

I am using the following animations (reimplemented with Ember JS), but failing to understanding how this CSS works. Refer to the link below.

http://ift.tt/2crMYZ2

#nav-icon3 span:nth-child(1) {
  top: 0px;
}


#nav-icon3.open span:nth-child(1) {
  top: 18px;
  width: 0%;
  left: 50%;
}

toggleClass('open') is triggered when the div is clicked, but but what is 'open' actually? Is it a subclass of nav-icon? And if it is possible with ember, would simply toggling between the two CSS blocks (open and not) show the animation?