ORP is oxidation reduction potential.
if you go back to your chemistry days in high school.
if you have an excess of oxidants (electron removers) in the water the value will be negative.
If you have an excess of reducers (electron Donors) in the water the value will be positive.
Basically if you have residual chlorine, ozone, bromine, flourine, or any other thing that will attack biological organisms then your ORP will be negative.
if you need any more info i can get if for you but 99% of my experience with it has been in industry and not in the aquarium hobby.