Just so you know "Iron Mask" is nothing more than a given level of discrimination on your detector.
Set the Iron Mask low say -30, and you have very little discrimination, which means you can detect both ferrous(iron) and non-ferrous targets in the ground. Set it high say -16, and you have very high discrimination.
Meaning most ferrous(iron) metals will be rejected, while most non-ferrous targets will be accepted.
Turn the masking feature off or open to -31 and you're hunting in All-Metal mode, no discrimination.
You have the ability to set how much/little discrimination you want with the Iron Mask feature on an Explorer.
Metal in the ground can be categorized into one of two categories: Ferrous or Non-Ferrous. Ferrous metals are attracted to a magnet, while non-ferrous metals(copper/silver/aluminum) are not.
Metal is also categorized by its conductivity: low or high conductors.
Low conductors are nickel, aluminum, foil, small gold items. High conductors are copper, silver, larger(higher karat) gold items, and Iron(ferrous targets).
With an Explorer you have a choice on how you want to hunt for metals in the ground with regards to the tone you will hear.
If your "Sounds" setting on your Explorer is set to "Ferrous", then your high ferrous(iron) targets will produce low tones, and your low ferrous targets will produce high tones. If your "Sounds" setting on your Explorer is set to "Conduct", your low conductors will produce low tones, while your high conductors will produce higher tones
Personally I have been a "Ferrous Tones" hunter for 12 years now and would never hunt in Conductive.
Hope this helps you better understand the "Iron Mask".