Toilet paper is made from paper wood pulp. The grain is generally very fine and short to allow the paper to disintegrate and break down faster in water. More expensive toilet paper can also contain small amounts of lotion or aloe for comfort. On the surface, that's pretty much it.
The concern comes from toilet paper made from recycled paper. Certain types of papers have a chemical called BPA (bisphenol A) added to them. Although not curtailed by the EPA, some people fear that BPA and similar paper additives might mimic estrogen and potentially be dangerous to hormonal systems. Since recycled paper products use paper from all different sources, toilet paper made from recycled paper can contain BPA as a result. Some people really don't like this since it's very difficult to determine the paper source of your toilet paper, but I suspect that the issue is mostly benign.