A siderophile (or siderophile element) is an element that is soluble in molten iron. This affects where it is found: if sometime in its history, it was in contact with 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 Earth's iridium dissolved in Earth's iron during Earth's formation and is now in the Earth's iron core. For this reason, the presence of iridium at one particular layer around the Earth is taken as evidence of a major meteor impact sometime in the past.
This solubility can depend upon pressure and temperature.