We can compare camera into eyes. Just like our eyes has lens camera has a lens as well to focus light onto an image recording area. In camera, an iris or the aperture is adjusted to control the quantity of light that is passing through. Keep in mind that over or under exposure of film or digital images may become completely black or white. Like the eye, the camera has a second light control which is the shutter. This will determine how long each exposure takes.
Doubling or having the shutter speed adjust the exposure by one unit is a measurement called stop. To capture the right amount of light and record the full range of stones in a scene the aperture and shutter speed must be combined correctly. So the measurement of light allows the photographer to arrive at the correct exposure. Too much light causes overexposure and too little underexposure when it happens even a small degree the image detail is lost which can never be recovered.
When it is underexposed, shadow details begin to form black blocks and the contrast drops if we use photoshop or printing techniques to adjust the levels to normal colors become weak and passed early and the picture looks grainy lacking subtle tones and fine details. When the image is overexposed, contrast increases and highlight areas become blocks of white, see the next article. As over and underexposure increases the white or black areas spread out and cannot be recovered.