Tempering is a method of combining two ingredients that are at two different temperatures, one hot and one room temperature or cold. It’s used when one ingredient (usually eggs) cooks at a low temperature or curdles easily. Imagine pouring hot milk or cream into a bowl of egg yolks. The extreme heat would cook the eggs, leaving scrambled eggs in your pastry cream, sauce or creme brulee.
To temper eggs, whisk a little of the hot ingredient into the eggs. It’s important to whisk constantly and vigorously as the hot ingredient is added. By keeping the eggs moving constantly, you raise the temperature of the eggs gradually, keeping them from cooking. Raising the temperature of the eggs protects them so that when the remaining hot liquid is added to the eggs or the eggs are added to the pot or pan with the hot ingredient, they won’t cook or curdle.