Training zones represent different exertion levels for physical activity and exercise.
Zone 1 - Light activity/exercise which has a minimal impact on heart rate. Walking falls into this category.
Zone 2 - Moderate intensity exercise which a noticable impact on heart rate. Breathing becomes heavier, but you can still talk without gasping
for air. Most structured exercise will fall into this category.
Zone 3 - Intense exercise which has a maximal impact on your heart rate.