Central heating radiators emit heat both into your room and towards the wall that the radiator is mounted on. If the wall is an external wall and not well insulated you can lose much of that heat. 50% of the heat emitted from a radiator is directed at the wall and approximately 25% of it is lost through the wall! 

Radiator foil is a thin sheet that reduces heat loss into external walls by reflecting heat back into the room. This is a quick, simple and cheap alternative to wall insulation and can reduce the heat loss through the wall down to about 2%.

It is important to use well designed radiator foil that uses aluminium foil and suitable insulation material in order for it to be effective. Google can help you find these but here is a link to Expert Review.