A solar oven works by concentrating solar radiation on the pot or utensil used for cooking. The sun's rays can come from different directions and the efficiency to collect that energy and transform it into heat is low, so aluminum foil is placed to help with this efficiency.
What aluminum foil does is receive those solar rays and reflect them directly to what we want to cook, reducing the loss of solar rays that do not go directly to the food, then it increases the radiation towards the food, making it cook much faster.
