When it comes to aluminum foil, one side is slightly more reflective than the other. The shiny side of aluminum foil is actually the side that reflects heat. This is because of the manufacturing process where sheets of aluminum are rolled multiple times under high pressure, causing one side to become shiny while the other side remains dull.
In practical terms, if you want to use aluminum foil for cooking or insulation purposes, placing the shiny side out will help reflect heat or light. Conversely, the dull side will absorb heat slightly better. However, for most cooking applications, it’s relatively inconsequential which side you choose to use, as both sides are effective insulators and can withstand cooking temperatures.