Older browsers will not understand a line of CSS that contains var(...) so they will ignore that line completely. In order to accommodate those browsers, you need two declarations. When there are multiple rules that a browser understands, it will take the last one, so we want the fallback to be before the preferred declaration. The "existing background declaration is the one that was already there in the challenge code:
background: var(--red-color);
“immediately before” means on the blank line above it.