Reflectors come in a variety of types. Popular is polystyrene plates in various sizes. They are cheap, lightweight and easy to use. And being white it will reflect the light very well.
They are often used to reflect light into the shadows of the subject, f.x. to light up one side, of a person's face, or under the chin.
Reflectors can be bought as foldable discs with several colours, mostly used are white, golden, silver, or black. Silver will reflect the light very well and harder than the golden colour, that then will have an effect of casting a warmer colour. White is softer than the silver reflector, and gives a nice soft reflection. Black will not reflect the light but keeping light from hitting at a chosen place in the frame - covering the part from being lit, an effect like a barn door.