A moment of inattention, a bad setting, and the temperature of an iron gets too high…… If you have used your iron at too hot a temperature, something may have stained the underside of the iron. In order to clean your iron, start by unplugging it and leaving it to cool down.
Apply next a paste of baking soda and water with a soft cloth on the underside aluminium and chrome part of the iron. Gently wipe it with the help of a damp cloth.
If the underside of your iron is made of Teflon or covered in metal, clean it with a nylon scrubber once it has cooled down.
And the next time, watch the temperature!