It looks like a stepping ring (allows the use of larger accessories such as filters and hoods). One thing I find with filters, stepping rings etc is that the harder you try to unscrew them, the tighter they become. This is to do with the manner in which you hold them - if you wrap your hand around the ring and grip it tightly this deforms the ring making it actually grip even tighter on the thread as you try to turn it. Try turning it but gripping it more loosely. Alternatively if this does not work another trick is to place the palm of your hand flat on the front face of the ring and turn the body of the lens with the other hand. If this does not work, try the same with something "grippy" like a mouse mat. Place the lens front down on the mat on a flat surface and press down. Now turn the body of the lens. This holds the ring in place without deforming it and allows it to unscrew more easily - remember you need something to hold the ring steady without deforming it as you turn. I have had to do such things many times with filters which can also sometimes become locked into place. Good luck but I am confident this should work - I have never had a filter locked so tight it will not come off using this technique.