Exposure is about how light and dark the image is. If you were below a rock and was to take a picture it would be under exposure however if you were above a rock it would be over exposure. That's why you need to have it in the middle for the perfect exposure. Having the perfect exposure means having the perfect lighting for the image. Here's examples of perfect, under and over exposure.
This is overexposure. This image is overexposed because the light is too bright and you can't really see what the image is.
Underexposure. Underexposed means it's dark and you can't see what the image is. ^^^^
This is a perfect exposure and it also has a large depth of field. This is perfect exposure because it isn't too bright or too dark.