Hi. When you measure the exposure for a scene using the camera's meter and it 'sees' the light reflected from the trees, grass sky etc etc.

The meter then tries to expose for this using 18% gray as a reference point. Anything which has a tone darker than 18% gray will be slightly overexposed because to the camera's and anything over 18% gray will be underexposed.

Now, take the same scene, but put the camera in manual. take a light meter reading from your 18% gray card. Because the camera's meter is looking at an 18% gray object and has 18% gray as its reference point there will be no underexposure or overexposure in compensation. So now black will appear black and white will be white.

For example, take a piece of white card, your gray card and some black card. Lay them on some grass, meter from them and look at the pictures. The white card should appear light gray, the black card will be a dark gray or off-black and the gray card is gray. Now do the same but using the manual meter reading from the gray card on each. You ought to get white = white and black = black.

Hope that made sense.

Matt