an important tool in light metering, it is just a cardboard card in matte 18% grey.
It is used for precision light metering, because all light meters are (or should be) calibrated at 18% grey.
Kodak sells them at outrageous prices. You can make your own by buying an appropriately hued piece of cardboard.
In many situations, you can substitute the palm of your hand.
Human hands are all more or less the same color on the palm side. This color is one stop brighter than the grey card.
This is why you meter off the palm of your hand, and add one stop. It always works, provided your hand is in the lighting condition of your subject.
And losing your hand is more difficult than losing a grey card.