That is where the yolk sac was and how the scar has healed. I didn't want to say anything on your other post, but now with this picture I will say, I noticed your tortoise looks like it had a very dry start. The way the yolk scar has healed certainly shows this even more. Not sure how the breeder started this little guy, but sure doesn't look like it had time in a brooder box to get started. Almost looks like a ground hatch or maybe just a very dry incubation medium and left in the incubation box dry as it absorbed the yolk sac??
But it is showing nice growth now, so that is a very good sign. Looks like it is now doing very well and with your care should be a great tortoise.