Titanium dioxide is a mineral that is commonly found in food coloring, toothpaste, paint, and sunblock. It absorbs UV light at a high capacity, it doesn't break down in the heat of the sun, it's gentle on the skin without leaving white traces and it creates a physical barrier against rays making it a sunblock, not just a sunscreen. Therefore it is often present in SPF for children and those with sensitive skin. Also, since it is encapsulated inside of a silicone molecule, the application is smooth and lightweight.
However, there is controversy surrounding titanium dioxide because of its possible link to cancer in humans. The argument for it is that it still blocks cancer-causing UVA and UVB rays, making it safer to use than nothing at all. For those of you who are looking for a titanium-dioxide free sunscreen until more conclusive evidence is released, check out these options.