Generally, flux is a substance added to a metal or metal oxide system to lower its melting point. Flux may be used to protect the surface of metal for annealing or soldering (welding) by allowing for a lower working temperature, thus lessening oxidation. The addition of limestone and lime in iron smelting charge has a slightly different purpose in that it reacts with impurities in the iron, forming a slag that floats to the top of the heavier (and now more pure) liquid iron.
Flux - Definition 
