Reflux is a technique used in chemistry to apply energy to reactions over an extended period of time.
A liquid reaction mixture is placed in a vessel open only at the top. This vessel is connected to a Liebig condenser, such that any vapours given off are cooled back to liquid, and fall back into the reaction vessel. The vessel is then heated vigorously for the course of the reaction.
The advantage of this technique is that it can be left for a long period of time without the need to add more solvent or fear of the reaction vessel boiling dry as any vapour is immedeatly condensed in the condenser. In addition as a given solvent will always boil at a certain temperature, you can be sure that the reaction will proceed at the same temperature; by careful choice of solvent, you can even control what that temperature is.
The diagram also includes an optional beaker of water between the reactants and the heat. This is often used as a safety precaution when using flammable reactants and a Bunsen burner in order to keep the flame away from the reactants.
see also:
flux