It means "sifted flour", that is, flour that has been sifted. So measure after you sift. I wouldn't worry about for most recipes, but I would stop tamping it down, though. If you have a fussy recipe, you'll end up with too much flour, and for most recipes it's not good to have packed down flour. The only thing that requires tamping is brown sugar because it's so affluous (made that up...) you want to eliminate air pockets. If you want to approximate the sifted flour needed, just start with slightly less than the required amount, and see if that works. I don't sift always when it calls for it, and nobody dies! :0)