This part snaps onto the Phantom's landing gear (slide it on at a 90 degree angle, then rotate it into the correct orientation once it snaps into place) and provides some extra length in both directions, making the quadcopter more forgiving/less likely to tip over and run the props into the ground when landing, particularly on soft/uneven surfaces like sand.  Two skids will add a total of only ~22 grams to the weight of your quadcopter.