The histogram is a graphical representation of all the Pixel values from the camera. The horizontal axis will always display the bit values from the camera. For instance, a 8-bit camera will show 256 entries. A 10-bit camera will show 1024 entries and a 12-bit camera will show 4096 entries. The vertical axis represents the number of pixels at that hue intensity (for colour cameras), or intensity (for mono cameras).
The histogram will also display the intensity value for Red, Green and Blue channel of a color camera. The Sharpness score and Frame Rate values are also displayed in this panel.
To enable the histogram, click the histogram icon under the preview window: