What does an "&" before a pseudo element in CSS mean?

That's LESS, not CSS.

This syntax allows you to nest selector modifiers.

.clearfix { 
  &:before {
    content: '';
  }
}

Will compile to:

.clearfix:before {
  content: '';
}

With the &, the nested selectors compile to .clearfix:before.
Without it, they compile to .clearfix :before.

A nested & selects the parent element in both SASS and LESS. It's not just for pseudo elements, it can be used with any kind of selector.

e.g.

h1 {
    &.class {

    }
}

is equivalent to:

h1.class {

}

Here is an SCSS/LESS example:

a {
   text-decoration: underline; 
   @include padding(15px);
   display: inline-block;

     & img  {
                padding-left: 7px;
               margin-top: -4px;
             }
 }

and its equivalent in CSS:

a {
  text-decoration: underline; 
  @include padding(15px);
  display: inline-block;
}

a img  {
     padding-left: 7px;
     margin-top: -4px;
   }