Flux paste
A pasty substance applied before welding or brazing to chemically dissolve oxides and prevent oxidation.
The job of flux paste is to dissolve oxides on the metal surface and protect the bare spot from oxygen until the end of the soldering or welding process. If you work without paste, the solder won't bond properly with the base material – you'll get poor wetting, but also bond defects and porosity.
Apply the paste thinly and only where you actually solder or weld. Too much is counterproductive, because the residue ends up as slag inclusions in the weld or causes corrosion. After cooling, you must remove all residues completely, especially on stainless steel – use chloride-free paste there, otherwise you'll get pitting. And stop smearing paste on an already hot workpiece; that only causes spatter and a mess.