How do you define, "Beat the yolks and sugar together until light"?

Hey Everyone - I'm making Crème Brûlée tomorrow (Sunday) for my wife's b-day celebration, following Mark Bittman's recipe from How To Cook Everything. I've made it MANY times before, always to rave reviews, but I've also never fully understood how to tell when the yolks and sugar beaten together equal "light." What should I be looking for as an indicator? I must be doing something right/okay based on how well it has always turned out, but I'd still like to better understand what "light" means - then see if it turns out even better. Thanks everyone.