Exposure is determined by the shutter speed and aperture; it is the quantity of light reaching the film or digital film (in most cases today it will be digital). If your shutter speed and aperture match with the right exposure, you would come out with a “perfect” photo essentially. In exposure you have “overexposed” and “underexposed”.
Overexposed is when there is a loss of highlight detail, meaning when important bright parts of an image are “washed out” or effectively all white, known as “blown out highlights”.
Underexposed is when there is not enough shadow detail, when important dark areas are “muddy” or indistinguishable from black. The photo is not lit properly, causing all detail to fall to the waist side, or known as “blocked up shadows”.
Perfect Exposure will look something like this photograph. You can see how it has perfect highlights along with the right amount of shadow, low lights. This brings out the right amount of the mountains in the back, still showing detail of the clouds in the mountains. It also shows the clouds at a perfect freeze in time, while smoothing out the “death” look in the grass (it being brown/dead like), and creating enough light to lighten the cabin just enough due to its dark exterior.