Low intensity is where your heart rate is 75% of its maximum. During the workout, your body will burn stored fat, provided your blood sugar is low and muscle glycogen has been used up. This only happens in two situations. When you first wake up and right after weight lifting. These are the best times to do cardio cause you'll be burning fat but not losing muscle. After adding morning cardio to my routine, I started losing more weight and inches and I feel better.
High intensity is where your heart rate is 85% of its maximum or higher. During the workout, you burn glucose but immediately afterwards it causes a massive release of stored fat. To ensure that the body uses this fat, you need to wait 30-60 minutes after your workout is done before eating. Weight training counts as high intensity exercise. I've noticed that I feel very pumped and energized after high intensity exercise and my leg muscles are more defined from all the running. Don't be afraid to go for high intensity. You won't have to work out as long and you'll still burn a lot of fat.