The &
tells the pre-processor to add the parent selector in-front of the nested class.
so this:
.class-name{
background: blue
&.class-name2{
background: red;
}
}
would compile too:
.class-name {background: blue;}
.class-name.class-name2 {background: red;}
The usage would be:
<div class="class-name">This background is blue</div>
and
<div class="class-name class-name2">This background is red</div>