Rule of Thumb: If total burned area is larger than a softball, get medical attention.
A quick physics lesson: Heat is a result of increased energy. Energy likes to flow from places of high energy to places of low energy until it’s equaled out. So heat will flow from the hot thing to the cool thing until they are the same temperature. Because the temperature difference between the hot thing and your skin is so large, the heat transfers really quickly, to the point that the water in the cells evaporates and causes cell injury.
Burns can lead to dehydration due to the loss of moisture from that area. Burns can also lead to infection due to cell injury and possible broken skin from blisters.