Those are breaks in the spines of the tail. They will never go away or grow out. There is one way to get rid of it but to some people this may not be an option for them. You would have to cut it out further up the tail and the new tail would grow back perfectly. I have done it myself before in the past and it works. Make a clean cut with a razor blade don't have to cut the whole tail just across the spines of the broken ones.
