A gray car is used when you're shooting something that is mostly black or white, which tends to fool your meter into over or underexposing. A gray card is basically put in front of the camera in the same lighting that your subject will be under and use the exposure that the camera gives for the gray card.
Like it was mentioned above, you can get a free card by calibrating your hand. Go to a shop that sells them, meter off the gray card, then meter off your hand. Remember the difference in the exposure (It should be 1 or 2 stops brighter) and then use your hand as a gray card from then on always remembering to overexpose that difference of your palm.
If you still don't get it, you need to read more about your camera's sensor and how it sees things in gray. Understanding Exposure has a section just on this.