Что значит символ $ в CSS

$color-dark: #000;
$color-light: #fff;
$bg-size-end: 4.2;
$dur: .3s;
 
html { box-sizing: border-box; height: 100%; font-size: 10px; } *, *::before, *::after { box-sizing: inherit; }
 
body {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  background: #57bd84;
}
 
input[type=checkbox] {
  height: 0;
  width: 0;
  visibility: hidden;
 
  &:checked + label {
    &:after {
      transform: scale($bg-size-end);
    } 
  }
}
 
label {
  outline: none;
  user-select: none;
  color: $color-dark;
  font-family: 'Lato', sans-serif;
  font-size: 2.5rem;
  letter-spacing: 0.04rem;
  padding: 1.5rem 3rem;
  cursor: pointer;
  border-radius: .4rem;
  border: .3rem solid $color-dark;
  background: $color-light;
  position: relative;
  overflow: hidden;
 
  &::after {
    content: '';
    position: absolute;
    width: 100%;
    height: 100%; 
    top: 0;
    left: 0;
    transform: scale(0);
    transition: transform $dur ease-in;
    mix-blend-mode: difference;
    //Gradient start values are somewhat arbitrary. But this seemed a good fit to avoid overly-blurry circle edge.
    background: radial-gradient(
      circle at center, 
      $color-light 24%, 
      $color-dark 25%, 
      $color-dark 100%);
  }  
 
  box-shadow: 0 3px 0 0 $color-dark;
  &:active {
    top: 3px;
    box-shadow: none;
  }
}