The solder flux transfers the heat from your soldering iron to the metal joints easily and makes the solder wire to flow like a liquid. You can also use this to remove the soldered part with the help of Desoldering Wick or a Desolder Pump.
Directions:
Apply little flux to the metal parts of the solder joint before soldering to get heat transfer for great clean and long lasting joints. Primary purpose of flux is to prevent oxidation of the base and filler materials.
Post-soldering flux residues do not cause corrosion of non-ferrous metals and may remain on the joints. (it is advisable to wash them with alcohol remover).