Hi Jao,
Great question, it's not well documented at the moment :) Right now, the way we visualize histograms is by showing where different %iles are distributed in the data. So the darkest line in the center shows the median of the data, the next line up shows the 68th percentile, the next line up shows the 95th percentile, the next line up shows the 99.7th percentile, and the final line (with the very faint shading) shows the maximum of the data (the 100th percentile).
We're planning to add a crosshair display that will annotate this so it's easier to understand. In the medium term, we want to shift away from showing percentile summary statistics, and show the actual probability density function instead. The percentile approach has a few disadvantages, for example if your data distribution is bimodal it does a bad job visualizing it.
Dan