At your age, you would probably see a noticeable improvement in your neck by achieving a weight within the normal BMI range. The fat in your neck will reduce, giving you a smoother contour.
What is this? I'm kind of chubby and I'm 14. I've had this as long as I can remember on my neck.
There is fat underneath the skin in many parts parts of the body. What you notice in your neck is fat, too. It is possible to remove the fat with liposuction but you are so young and still growing. Wait and see how things may change over the next few years.
This is probably fat but if there is any firmness or asymmetry to the area I would definitely get it checked by your doctor. Better to be safe than sorry.
From the one view it is difficult to say but I would bring it to the attention of my family MD and get his opinion. That said it appears to be fat