This zone is the least intense. You are in it just to warm up or cool down, in order to prevent injuries.
Stay in this zone if your goal is to lose weight. You body is fueled by 10% carbohydrates, 5% protein and 85% fat. The actual number of calories burned per minute will be less than in higher zones, but you'll be able to maintain exercise longer.
Also called aerobic zone. In this zone you'll get the greatest health benefits overall. Your heart and lung capacity will increase and your overall fitness will also increase, so you'll be able to exercise longer. Your body will get 50% of your calories from fat, 50% from carbohydrates, and protein burn will be minimal.
Also called anaerobic zone. This is a highly intense exercise and should be limited to no more than 10-20minutes. Your body's ability to consume oxygen will increase. If you push it too hard your body will start producing lactic acid. Your body will burn 85% carbohydrates, 15% fat, and minimal percentage of protein.
You are pushing your heart to the limits here, and should consult your doctor to check if you can exercise in this zone safely.
Heart rate zones in this app are calculated from Karvonen Formula.
The Karvonen method factors in Resting Heart Rate (HRrest) to calculate Target Heart Rate (THR), using a range of 50%–85%:
Example for someone with a HRmax of 180 and a HRrest of 70:
50% intensity: ((180 − 70) × 0.50) + 70 = 125 bpm
85% intensity: ((180 − 70) × 0.85) + 70 = 163 bpm
More info is available on Heart Rate page on Wikipedia