What is the histogram? It is a visual representation of your camera’s dynamic range. On the left, you have your blacks, on the right, your whites. In between all the tonal variations of the image you’re looking at.
The histogram is objective. No more guesswork, looking desperately at your LCD display hoping that what you see there, is actually what you are getting. By now, you shouldn’t be surprised if I tell you that the LCD display is nothing more than an approximation of the final image. And if you’re shooting outdoors with your DSLR, you can’t even see it properly. There’s so much ambient or direct light, that you won’t perceive shadow detail at all. You might end up thinking that your image is underexposed or even worse, you might overexpose it.
Every cinematographer needs an objective point of reference. Even the keenest eye can be fooled or grows tired. This point of reference used to be a light meter. These day, with the dynamic range of digital cameras increasing all the time, it’s more and more the histogram.