Histogram tool show current displayed image color histogramm in real time. The histogramm is displayed for currently selected layer. For RAW images the histogram is calculated after their transform to 8 bit.
By selecting "L", "R", or "L+R" you can control which stereo frame is used to show the histogram.
Z, S, values show number of pixels with zero and saturated values. "C" value is image contrast measure (max - min / 255).