In cooking, reduction is the process of thickening and intensifying the flavor of a liquid mixture such as a soup, sauce, wine, or juice by simmering or boiling. Reduction is performed by simmering or boiling a liquid such as stock, fruit or vegetable juices, wine, vinegar, or a sauce until the desired volume is reached by evaporation. This is done without a lid, thus enabling the vapor to escape from the mixture. Different components of the liquid will evaporate at slightly different temperatures, and the goal of reduction is to drive away those with lowest points of evaporation. It thus can be seen as a form of distillation, capturing those components that have the highest boiling point.
While reduction does concentrate the flavors left in the pan, excessing heat can drive away volatile flavor compounds or even juice from the meal, leaving behind dryer food lacking in essential flavors.
^Sauces from basic brown sauce to Béchamel sauce and even tomato sauce are simmered for long periods (from 1 to 10 hours) but not boiled. Simmering not only develops the maximum possible flavor, but also allows impurities to collect at the top and be skimmed off periodically as the sauce cooks. Boiling diffuses the impurities into the liquid and results in a bitter taste and unclear stock. Source: The Culinary Institute of America, The Professional Chef, 9th edition (John Wiley & Sons, 2011) pages 268-70 (brown sauce), page 295 (Béchamel Sauce and Tomato Sauce).
^See also Martha Holmberg, Modern Sauces (Chronicle Books, 2012), pages 199-205 and 236-38 (sauces that are simmered, not boiled). However, she also has recipes in which sauces are boiled, pages 241-42.
^The importance of simmering sauces is illustrated also in America's Test Kitchen Cooking School Cookbook (America's Test Kitchen, 2013), page 232.
^Broths are also simmered rather than boiled, and for the same reasons. The Culinary Institute of America, The Professional Chef (John Wiley & Sons, 2011) pages 304-05.
^This is true even though some cookbook authors unwisely use the term “boiling” as a general term that fails to recognize the harm of boiling in preparation of many reductions such as stocks, sauces, etc. See, e.g., Sharon Tyler Herbst and Ron Herbst, The New Food Lover’s Companion, 5th edition (Barron’s, 2013) page 627.