Flux (from Latin "to flow") is a substance that facilitates the flow of solder by preventing the formation of oxides. Flux prevents oxides from forming by being more reactive with oxygen than the metals and solder that the flux is applied to. Generally flux is combined with water to make a fluid that can be painted or sprayed onto the workpiece. When heat is applied the water evaporates to leave a glassy coating. As oxygen and other elements combine with the flux it becomes less effective.

--Partially excerpted from The Complete Metalsmith by Tim McCreight.