Hydrogen peroxide dosing is used typically to treat algae. Dinoflagellates, cyanobacteria, etc.
Very short answer on the chemisty. As said above, it is a strong oxidizer. In chemistry this means it has a strong potential to pull electrons from other things.
ORP, mentioned above, is the oxidation reduction potential of your water. In essence, how likely your water is to break down contaminants. Dumping a lot of hydrogen peroxide into your tank will make it have a very high ORP.
.... The thing you need to worry about, is that your fish, corals, and livestock are 'contaminants'.
The goal of dosing H2O2 is to dose a small amount that will have a negative effect on the floating suspended algae and finer, filamentous algae that are not as 'sturdy' as your corals and living creatures. Think of it as sterilizing the water... but only just enough.
That is why you will see most guidelines telling you to put millilitres of H2O2 into your tank per gallons. A very small amount!