These are both easy and tough questions. In the most basic sense, exposure is a measure of how much light is detected by your camera's sensor. "Perfect" exposure is one in which there are no areas which are "too dark" or "too light". Of course, that is a subjective determination in most cases. But from the point of view of the camera, there are ways to determine if parts of the picture are over or under exposed. This is done with the magic of the histogram.
Many cameras (and most, if not all DSLRs) have a capability to display the histogram of a captured image. The histogram is a graph of how much light has been detected by the sensor in several brightness frequencies. A mouthful of stuff there, but the picture is worth a thousand words here.
The left part of the histogram is a measure of how much darker area there is in the image while the right side is a measure of how lighter area there is. The center is a measure how much "midtone" brightness exists. Ideally, you would like the histogram to have no areas where the level hits the top of the graph. That would indicate that there is too much information in that brightness band for the camera to process it successfully.
For example, if there are levels on the left of the histogram that reach the top of the chart, those areas are said to be under
exposed. Likewise if levels on the right of the chart reach the top, they are said to be over exposed. This histogram shows the levels on the right side to reach or exceed the camera's ability to successfully process the image.
After a while, you will see that the images with that balanced exposure histogram are more pleasant to your eye than ones that are over or under exposed. Of course, the composition of the image is not addressed with this ... only the balance of brightness.
This balance is achieved by using the "Triangle of Exposure": Aperture, Shutter, and ISO.
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