A reflector is a kind of lighting modifier designed to reflect light back towards your subject. Usually it’s a white or foil-covered disc stretched tight across a circular wire rim. When the reflector’s not being used it can be twisted into a much smaller disc and stored in a circular carry case.
If you are shooting a portrait with a single light, for example, you can use a reflector to bounce light back into the shadowed side of your subject’s face. Or if you are outside in bright sun, you can use a reflector under your subject’s face to bounce light back and reduce harsh shadows.
Reflectors are usually circular, but may be rectangular. They come in different sizes and surfaces. Some 5-in-1 reflectors come with slip-on covers with different finishes for different effects, e.g. gold foil and silver foil. Some can also act as a diffuser for a harsh light source.