When I moved into a new house out of my old neighboorhood (Where I was very active), I gained about 10-15 pounds (Thats not such a big deal), I also just sat at my computer all day and sloutched (Where you bend over half-way while you sit (The opposite of sitting up straight

)), same in school. So, my stomach got in a couple rolls now I have 2 marks that go across my stomach, sometimes they fade and then they will be there. Even when they fade you still see where they were a little (Like the marks/creases on your palm, to sum it up)
What are these marks exactly, how do I get rid of them?