^They are rubber band loops that you can use to add or subtract resistance from an exercise. I use them choke hitched around the bar and then I step into the loop that hangs down.
Depending on the make, greens are 45mm wide and give approx. 10-25kg resistance/assistance dependent on set up. Purples are 29mm, 2-22kg. My blues are 21mm, 2-16kg of tension.