The histogram
The histogram (a generic term, taken from statistics, but used in digital photography,) shows the relative number of pixels at each level of "brightness" from black to white. The horizontal axis shows 256 levels of brightness: 0=black, 256=white, 128=gray. The vertical axis shows the relative pixel count at each level. The "perfect" histogram of a balanced scene would look like a bell curve.
Below is a screen capture from PhotoShop, showing the histogram for a rather challenging photograph. Notice that the histogram is "balanced" even though there is little information at any light value other than very dark and very light. The leaves contain all the intermediate values. The image is well-exposed.