A picture is ‘correctly exposed’ when the tones you are most interested in are clearly reproduced.
The camera needs a certain amount of light from the scene to produce pictures with good tonal gradation, color fidelity, and little picture noise.
If it receives too little light – because of insufficient illumination, or the lens aperture being too small (stopped down), the picture becomes underexposed. AH tones appear unduly dark, shadows merge lifelessly, picture noise speckles darker tones, and spurious effects develop (e.g. lag, shading). You can’t compensate for bad under-exposure by increasing video gain. It just makes the picture brighter, and defects more noticeable.
When the CCD receives too much light, ...