Heart Rate question

I do cardio for about a half hour and burn 500 calories. However at the end of the workout when i end my HRM and it gives me my stats, it says I was in the zone for 7 of those minutes. But I still burned 500 calories. What does it mean to be in the zone. I dont know anything about Heart Rate combined with fitness and weight loss. Is there a website or artivle to explain it to me better? I hope i am not messing anything up.