A long time ago some smart people determined that the tone midway between black and white was the perfect standard to use in photography. This tone, appropriately named middle gray, is essentially the color that cameras are trained to see. In other words, when you click the shutter your camera exposes the scene as though the scene was painted entirely with middle gray paint.
But what happens when the scene is anything but middle gray? A black cat at night, for example. Or, a polar bear in the snow. Your camera is fooled in these situtions and will likely incorrectly represent the actual scene.
The same often occurs when using a camera for photographing art work. If the painting or sculpture being photographed is anything darker or lighter than middle gray the camera will make an incorrect exposure.
This is why gray cards exist. A gray card is a tool used to fool your camera into thinking it's looking at an average scene, a middle gray scene.
So, whenever you have reason to suspect your camera may be fooled a five dollar gray card will quell your fears. Here's what you do:
1) Under the same lighting as your subject hold or mount the gray card in front of the camera so that you see nothing but the gray card when looking through the viewfinder
2) With the camera set to "manual exposure" change the aperture and/or shutter so that meter indicates a "0" on the scale.
3) Take note: if your lighting does not change and your camera remains in relatively the same position, the exposure reached in step is 2 is the proper exposure regardless of what is placed in front of the lens there after.
4) Fire away.
An 8x10" gray card shown on top of a large inkjet print.