Hello~I have some similar markings my self, but they are under my breasts. The doctors that have looked at them have said they are not serious, probably a type of fungus, and sometimes washing the area with the shampoo called Selsun blue with help to get rid of them.
How long have they been there? If they have been there a while, you should see your dr. It could be something serious, but most likely not. It could also be a chaif mark from something rubbing the area a lot. Ring worm is another possiblility not knowing what the spot looks like.
I hope that helps you.