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A grey card is exactly what it says it is. A card that is gray.

A grey card is designed to help photographers to adjust their exposure and white balance settings consistently by providing a reference point

The grey card works because of its lack of color. Grey is a neutral tone, and even though there are many different shades of grey, the grey used on a grey card is considered to be a “middle grey” or 18% grey. The middle grey hue makes it easy for the camera to understand a given lighting situation and prescribe the best solution.

FOR EXPOSURE:
More specifically, it is middle gray, or 18% gray. This number is important because 18% gray is what your camera’s meter is trying to calculate when it meters to expose for a scene. This means if you put a grey card in front of your subject, and take a meter reading off it, you will get a correct exposure regardless of any tonal contrast in the scene.

FOR WHITE BALANCE: 

When used in conjunction with a camera’s custom white balance feature, you can manually set a perfect white balance for everything you photograph.