A liquid reaction mixture is placed in a vessel open only at the top. This vessel is connected to a vertical 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. 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.