Hey Travis, George is right, in the manual pages 45,46 will help you understand how iron mask works, read it more then once then once again.
How it works is like George said, turning iron mask on allows you a choice as to how much iron targets you want to hear (reject iron or accept more iron). At -16 you are accepting all iron, at 0 you are rejecting all iron, you can ajust from 0 to -16 as to how much iron you want to hear, remember 0 less -16 more.
Why people use iron mask is simple, the key to finding silver coins and other goodies laying in the ground next to nails and bolts and gas caps is you have to get pass them to hear the good target laying next to them. Iron will mask out any good target laying next to it, so if you reject all iron 0 you won't hear the iron anymore but at the same time you won't hear the goodies either because the iron is masking out the good target. Now if you accept more iron lets say -10 you will hear the iron and maybe the good target laying next to it also. There are silver coins laying next to iron that are being masked out by the iron and in order to hear the silver coins you must hear the iron and hopfully doing so will allow the sound of silver to come through also. Hope this helps, read the pages in the manual Travis it will clear some of it up better then I can, good luck, hh
Ray ~