It’s hard to see in the photo. It looks like the belly button did not close up all the way like they should when young pups. This often leaves a roundish like lump ranging in size called a hernia. It looks there is an opening with some skin is sneaking out with hair - hard to see. Often these hernias are not serious but the vet would have to determine that. If this is something new and your dog is not acting himself or area is sore -I would get him to vet today just to be on the safe side.