Reflectors are used to bounce the light from a source onto your subject and are especially useful outdoors, where electrical power and supplemental lighting are not available.
A reflector can be as simple as a sheet of shiny poster board or foam core or as sophisticated as special material stretched over a frame.
A white surface will reflect a soft white light, a metallic surface (such as aluminum foil) will reflect a harsh light. You can also use colored surfaces as reflectors to achieve some interesting effects.
One of the most easily used reflective surfaces is the ceiling or wall in the room in which you are shooting.