So communes in China were encouraged to produce steel during the Great Leap Forward to help with the modernisation of China.

However, many (but not all) communes lacked the ability to produce steel straight up. Sometimes due to equipment, other times due to a lack of expertise or materials. These small scale blast furnaces (think of your classic medieval RPG forge/furnace, with coal, ore, flux and a "blast" of pressurized air from a blower of some type) would reliably smelt copper from copper ore and "pig iron" from iron ore, which is a brittle high carbon iron, much weaker than cast iron or wrought iron. Also, this method was fairly archaic, it's a literal pre-medieval technique as the first step for producing steel

While many critics point to this as a policy failure because many contemporary steel mills in the west and USSR could smelt iron ore then work it into steel without an intermediate step, en masse and electric, pig iron itself isn't a wasted or useless product because it can then be worked into wrought iron or steel at a higher temperature furnace.

This was all roughly 5-6 years before China's first successful nuclear bomb, as a frame of reference.