Another way to think about histograms is: Pixel Count.
The left side of the histo is 'dark', the right is 'light'. The camera goes through the image, and counts how many dark, medium and light pixels there are. It then creates a graph showing you how many dark, medium and light pixels are in the photo.
In a perfect world, there would be no pixels at or near either end of the graph. All the pixels would fall between the two ends. But that is something that is difficult to achieve in the natural world (studio-type setups are exempt from this).
For an 'average' scene, you want the bulk of your pixels to fall as close to the middle of the histo as possible. "High key" and "Low key" shots are an exception.