A Siderophile (or Siderophile Element) is an Element that is soluble in molten Iron. This affects where it is found as sometime in history, if such a material is near molten iron, it is likely to be mixed in.
For example, one siderophile, Iridium is rarely found on Earth, though common in meteorites. It is presumed that the bulk of the iridium on Earth dissolved in the iron and is in the Earth's iron core. For this reason, its presence around the Earth at one particular layer is taken as evidence of a major Meteor Impact sometime in the past.