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Temper can refer to two different cooking practices. The first meaning refers specifically to the process of heating and cooling chocolate to form crystals. This assures that the chocolate will be firm at room temperature.
The second meaning refers to a process most commonly used when cooking with egg yolks, as when making custard. When adding a hot liquid to egg yolk you must temper them first to avoid cooking or curdling the yolks. A small amount of the hot liquid is whisked into the yolks before the yolks are added to the remaining hot liquid.