Zone 2 in many training models is typically called the "endurance" zone. It represents the level of effort where you could do something for hours at a time. It is typically defined in terms of heart rate (beats per minute), running pace (minutes / distance), swimming pace (minutes / distance), or cycling power (watts). Usually you'll see something between five and seven zones defined, ranging from an active recovery effort (zone 1) all the way up to a zone for training anaerobic strength.