Rosin flux is a soldering aid made from pine tree resin that cleans metal surfaces by removing oxides, prevents re-oxidation, and helps solder flow and adhere to the metal by reducing surface tension. It becomes active at soldering temperatures, and its residues can be non-corrosive and non-conductive, though activated formulas may require removal. Rosin flux comes in various forms, including liquid and paste, and can be mixed with solvents like isopropyl alcohol for application.