Orp = oxygen reduction potential.
This is a very complex subject. ORP in a reef aquarium is a struggle or war going on in your tank. ORP is a measure of who is winning and who is losing this war. The two sides are oxidizers (
ozone, hydrogen peroxide, nitrate and many more ) and reducers ( ammonia, iodide, and sulfide ). If either side wins the war, your tank and all in it will die. Like most things in life, it's all about balance !
( Hope I didn't confuse you more than needed )
So I recommend just getting some good test kits for Ammonia, Nitrite, Nitrate,
calcium, Alk, PH, Phos and Mag. to start with, and worry about ORP probes and meters later on !