What is that stuff? Mine is chipping off. I can see painted metal in spots. Can this be resprayed from a rattle can? I assume so but what do I respray with?

your car was likely undercoated, you can buy spray on rubberized undercoating in a rattle can, or you can buy it in liquid form to be sprayed from a compressor and gun setup. Most auto parts stores would carry both forms.

does it hurt to not have it besides being aesthetically displeasing?

if its the bare matel showing most likely it will rust.

it's not bare metal. it's painted metal if that makes a difference.