Reflective materials are useful when growing in an enclosed area. Light from above will be wasted to an extent from other sides of the plant if it is not reflected properly.
One option for reflective material is biaxial oriented PET film. Better known as Mylar. It is a metallic film sold in the roll that will reflect light back on to plants. It is aluminized by evaporating a thing film of metal on to the sheet. The result is much less permeable to gasses and is often used in food packaging but is very useful in indoor plant growth.
Another option for reflective material is to use black and white plastic. These are sold in rolls similar to mylar only instead of a metallic coating, one side is white plastic and one side is black. This is very useful when a light source is outside of your growing area. Place the black away from plants and the white side towards it. The black will absorb light from the outside and the white will reflect it back on to plants inside your plant growing area.