The histogram display simply shows you where the brightness in your image “falls” within the 14-stop range. It is useful when you are shooting subjects that are predominantly white (like a bride in a wedding dress) or black (like portraits of black cats on black backgrounds), and you need to know if the sensor is capturing the detail that the LCD cannot show you.
It’s also doubly useful when you’re reviewing your images outdoors on a bright day and your LCD screen is getting washed out. Being able to see what you captured graphically can be a stress-reducer out in the field! The histogram shows you the range of brightness values in your image, rearranged in order, with the most frequently-occurring brightnesses being taller.
Left edge is the DARKEST value the camera sensor can capture.
Right edge is the BRIGHTEST value the camera sensor can capture.
When reviewing a histogram for any image, any part of the image that is “off the scale” or blown out will blink.