Iron ores are minerals which contain large enough fractions of iron for it to be economically extracted. On Earth, this typically means iron oxide formations with between 20% and 60% iron by mass.
Hematite, Fe2O3, is a chemical compound that is very common on Mars, and is responsible for the planet's red colour. However, the purity may not be high and so any Martian colonists may have difficulty producing quality metal (at least initially).
Iron ore for in-situ iron and steel production can be separated and concentrated by magnetic processing and gravity separation after milling to suitable grain size. Native iron is also available as meteorites on the martian surface.