font-awesome/free/scss/_icons.scss

// specific icon class definition
// -------------------------

/* Font Awesome uses the Unicode Private Use Area (PUA) to ensure screen
readers do not read off random characters that represent icons */

@each $name, $icon in $fa-icons {
  .#{$fa-css-prefix}-#{$name}::before { content: unquote("\"#{ $icon }\""); }
}