CSS PROBLEM: can anyone explain me what this code means? &HOVER? what does "&" indicate?

&:hover { .line-1, .line-2, .line-3 { background-color: #fff; } }

&.active {

.line-1,
.line-2, 
.line-3 {
  background-color: #fff;
}


.line-1 {
  animation: animate-line-1 .7s $cubic-bezier-in forwards;
}
.line-2 {
  animation: animate-line-2 .7s $cubic-bezier-in forwards;
}
.line-3 {
  animation: animate-line-3 .7s $cubic-bezier-in forwards;
}

} }