Solar ovens are a way to use the sun's energy to cook your food. Because they use solar energy, they can be less costly to run and energy efficient. Users don't need to pay or collect fuel. Pots and pans don't get as sooty, so they are easier to clean, and cleaning takes less water. There are also safety advantages -- fewer toxic emissions, can be safer around children.
There's a wikia on solar cooking. It has extensive information.