There are a couple of different ideas going on in a solar oven. The first, is using the foil to collect more sunlight. The foil that you use is intended to reflect more of the sunlight into the box. The bigger you can make these panels, the more light you can get into your box, and the hotter it will get. This is the same basic idea as shining light through a magnifying glass. As you probably have done at some point, if you use a magnifying glass to focus sunlight to a single spot, you can get that smaller area much hotter. Using the foil is the same idea.

Now, the second idea of a solar oven is having something absorb the light. Any object you put in there will absorb some light, so having increased the amount of light entering the box, you can make it get hotter. Some types of ovens will put dark (black) objects into the oven, because dark objects absorb the most light and don't reflect as much back. This means that dark objects will get hotter than light objects. In these types of ovens, sometimes food might be set on top of a dark object, or even put inside of it, to try and get it to a higher temperature.

Foil is reflective (being metal), and light is energy.In trapping the food inside a region where only it can absorb the light, you are focusing the light of the sun on the food over a much greater area (i.e. the box) than the food itself.

Basically, it's focused using mirrors. The idea is the same as using a magnifying glass - you're focusing energy on a small area. Archimedes developed mirrors as a weapon that could set ships on fire using this principle.