Radiator reflectors are shiny sheets that sit behind your radiator. They are designed to prevent heat radiated from the radiator becoming lost into the wall by reflecting this heat back into the room.
The heat radiated from a radiator travels in infrared waves. These waves are small enough to become absorbed by the wall. Reflector foils have a much smaller lattice structure than a wall so infrared waves are reflected rather than absorbed. Ordinary tin foil also has this property but would not be as effective for as long as radiator reflector foils because the surface of tin foil oxidizes on exposure to air, meaning over time it will lose the shiny surface and its reflective properties.
Radiators only radiate 20% of the heat they give out. Of this small percentage, 10% goes out of the front, where you would want it to go and the rest radiates from the back of the radiator. This is the heat that has a chance to be reflected by radiator reflector foils. But what about the other 80% of heat? That heat is convected. It rises from the top of the radiator and gets trapped at the ceiling. A Radfan can help solve this issue by sitting on top of the radiator and redirecting heat into the room. If you want to recover as much heat as possible from your radiator, the Radfan and radiator reflector foils, such as Radflek or Heatkeeper, work well together.