Original Extract refers to the concentration of sugars in the wort before fermentation begins. It is expressed as the mass of sugars in 100 grams of wort. This measurement is crucial for brewers as it helps in predicting the potential alcohol content of the beer. OE is typically measured in degrees Plato, Brix, or as dissolved solids. It can be measured directly using specific types of refractometers and hydrometers or calculated from the Original Gravity.