What does it mean if the main peak of my Histogram is beyond the top of the chart?

I tried reading this thread about histograms, but couldn't understand it well enough to answer this question.

I took some shots yesterday where the main peak of my Histogram was actually off the chart. The histogram had one large peak and most everything else was very low. What exactly does this mean, and if it's something bad (I expect that it is) how can I avoid this in the future?

Here is the Histogram in question :

and the image can be found here. (Update: I altered the photo, so the link is taking into account Jrista's excellent suggestions. This histogram no longer corresponds to this photo.)

Answer

The height of each individual, vertical line in a histogram indicates how much of that particular tone your image has, or the tones intensity. As tones in a histogram progress from left (black and shades) to right (brights and highlights), where a particular vertical line lies indicates how intense the tones represented by that line are.

If one particular tone is extremely intense, and the rest are very low, then the image is primarily comprised of that (or those) particular tones. Generally, this happens in the highlights when they get blown out. It it happens somewhere else, then the histogram is simply telling you that your image is almost entirely composed of that particular range of tones.

It should be noted that the histogram generally does not show enough vertical lines to represent the entire tonal range of an image. Every individual vertical line in a histogram usually represents a small range of tones.