Because of personal interest, I've looked up the general use to determine the relevance for Hydroponics but couldn't detect anything actually relevant (from my understanding of things). ORP seems probably more useful with

Aquarium Monitoring
Drinking Water
Swimming Pool Water

The use is either to measure ion activity, or to determine the oxidizing or reducing properties of water or of a solution. In other words it tells you how alive or "dead" your water is and this is determined by measuring electron present in water or a solution.

High +mV values (+) 300-450 mV indicate low electron presence, dead or highly damaged water, while low positive, lower negative mV indicate Ok-, and finally up/down to (-)120 to (-)130mV very high electron presence and very "alive" water.

Does it help me or you if I modestly admit that I am no expert, but hopefully only figured it out minutes ago? ;-)

I looked into this not long ago when looking up info on Milwaukee meters. What I was looking for was unrelated to ORP, but I was curious when I saw those meters in the catalog and looked a little further. I am not experienced at all either, but my understanding is that for home hydro, it isn't going to be of much use unless you intend to use ORP readings to control pH. I read somewhere about people using ozone to affect ORP and thus reduce pH. It was a commercial set-up, though.

Honestly, this is the first time that I have heard about that term, but I did a quick search for oxygen reduction potential and it looks to me that it is related to the dissolved oxygen levels of the nutrient solution. I am not sure if my understanding is correct, but I realized that Lucas may be right when he said that this is just a process that tells you how "alive" or "dead" your water is...For a while I thought it has something to do with TDS....So, it has really no great impact in hydroponics.

The thing about the benefit of how "alive" or "dead" some water is, is circumstantial. Means that with an aquarium you most probably want your water alive to provide a adequate biotope. And with a pool you don't want it exactly alive and teaming with all kinds of pathogens. And with hydroponics it is circumstantial. If you run a "sterile" way of growing, you don't want it alive- but if you go with beneficial microorganisms (some call it organic) and fungal inoculation, you would prefer having it rather alive, I guess.