1 cup of sifted flour means that you first sift the flour, then measure. So place a one cup measure on top of a piece of foil or waxed paper, grab your sifter, put that on top of your measuring cup and sift the flour directly into the cup until it fills the cup. Level that off and put it into a separate bowl, and repeat until you have measured out all of the flour for your recipe. Be sure to pick up the spillover excess flour on the foil and use that too because that has been sifted! Pour that excess into your measuring cup or if you've sifted all the flour you needed, simply return it to the flour canister.
1 cup of flour, sifted, is measured totally different. This means that you scoop flour from your canister using a separate spoon or scoop and placing that flour into your one cup measure, level it off and then place that one cup of flour into your sifter and sift it into a bowl underneath.
It really makes a difference in baking so be sure to carefully check the directions and when passing on a recipe of your own, be sure to indicate which method you used!
When using older, vintage cookbooks, you will often have to make a judgment call, because very often when they say 1 cup of sifted flour, what they actually mean is for you to measure out first, then sift.