These are the little dots on your arms which remind you, and anyone else looking at them, what your hobbies are. In a perfect world, there would be no tracks, because you would carefully wipe the hole where your hobby just went with a nice alcohol pad, then put ice on it to keep it from bruising. You want to avoid bruises at all costs because this tells people more about you than you want them to know.
However, since this is not a perfect world, things sometimes get out of hand. You chuckle to yourself as you look at a particularly sinister dot, right there in the crook of your elbow, where you almost lost a very important piece of the narcotic puzzle one night, long ago. Remember? You had set the needle very carefully and the blood was drawn to make sure you were correct. Then, just as you plunged the hobby into your vein, a knock came upon the door. You lost the vein and then were left with too much blood, along with your hobby, in the needle. This is dangerous, and can lead to hepatitis or worse if you don't get that hobby of yours back into the vein quickly. So you forgot about who might be at the door and began stabbing yourself fitfully in order to achieve the fun your hobby afforded you.
Some have found enjoyment in model trains. Others collect stamps. They perhaps have their own reminders of the little things in life that brought them joy.
You? You have the dots on your arms. The good news? They never go away. The bad news?
They never go away.