The act of tempering is done by gradually increasing the temperature of one recipe component by the addition of another.
To achieve this gradual increase of temperature, you drizzle the hot component into the cooler component while constantly mixing the cooler ingredient. If your container is sufficiently large, you may continue adding the hot component to this container, else you should bring the temperatures as close as you can, then whisk/mix the cooler components into the hotter.
Tempering is often done where eggs or yogurt are used as a thickening agent (i.e. in custards and sauces), since a sharp increase of temperature will cause the eggs to cook prematurely resulting in a lumpy texture or yogurt to curdle.
The same principle might be used when the addition of one recipe component might rapidly change the other, such as adding a large quantity of something acidic to something containing milk products.
Indian Usage edit
Tempering may also refer to the chaunk or tadka, a process used in a number of Indian dishes such as Tadka Dhal. This process refers to deliberately browning whole spices in very hot oil, which is then used to either fry ingredients for the dish or to garnish the dish immediately before serving. The process of tempering roasts the spices, liberates essential oils, and enhances the aromatic quality of the dish.