The Lightroom Histogram lives in the upper right hand corner of your screen, just below the Module pickers. And if you've never seen it before it can be a bit of a surprise and somewhat confusing as to what it is. There's a lot going on in a small space.
So what are those peaks and valleys of color? Well, it is a kind of map of the colors in your image. The Red, Green, Yellow and Blue represent those particular colors and the Gray represents where those colors overlap. The Histogram also represents the Tonal Value of your image with the darkest tones being on the left and brightest tones being on the right. Those Tones are broken down into 5 categories that are also represented in the Sliders of the Basic Panel - Exposure (middle of the Histogram), Blacks (extreme left of the Histogram), Shadows (mid left of the Histogram), Highlights (mid right of the Histogram) and Whites (extreme right of the Histogram).
So if we use the Histogram above as a reference we can see that there is a lot of Green, Yellow, Blue and Red information in the Shadows of the image because there is a "peak" of all those colors on the left side of the histogram. We can also see that there is more information in the highlights area but less information in the midtones or Exposure section of the Histogram. So the main function of the Histogram is to give you a visual reference of the tonal values in an image. Generally speaking, you don't want a Histogram to be weighted too much to the left or to the right and you also want to avoid dramatic "peaks" - again generally speaking.
In the original image above, you can see via the Histogram that there is a lot of tonal information in shadows (left side of Histogram) and good amount of tonal information in the highlights (right side of Histogram). And looking at the image itself you an see how the Histogram corresponds to tonal difference in the image between the dark foreground and the bright sky. You can see that there is little information in the midtones of the image - it's mostly a bright or dark image.
After making some adjustments we can see in the Histogram that the "peak" to the left (shadows) is much less dramatic as the shadow area of the foreground has been brightened. You can also see that more information has been pulled from the highlights and into the midtones of the image. Overall the image has less of a contrast between darks and lights and is more aesthetically pleasing. There is a blue spike in highlights which represents the brightness of the clouds but in this image I am not worried about that.
The Histogram also displays your camera settings for the image you have selected and in this image we can see that my ISO sensitivity was set to 100, my lens was a 10mm, the aperture was at f11 and the shutter was open 8 tenths of a second. The Histogram will also indicate if you are working on an Original Image, Smart Preview or Original + Smart Preview. Tapping on the Smart Preview lettering will open a dialog box asking if you want to delete Smart Previews. If you ever see a lightening bolt in the lower right of the Histogram panel that indicates that your image is not using the current Process Version and if you tap the lightening bolt it will ask if you want to update to the current Process Version(Version 4 is most recent).
The last thing I want to talk about about in the Histogram is clipping. In the upper right and left hand corners of the Histogram are triangles, as shown below. By tapping on the triangle to the left Lightroom will show you which areas of the shadows are being clipped - no information, just pure black - with a blue overlay. By tapping on the triangle to the right Lightroom will show you which areas of the highlights are being clipped - no information, just pure white - with a red overlay. You can have them both on at the same time or on individually and tapping the "J" key will turn on and of the "show clipping" function.
A section of the Histogram showing the "clipping triangles"
So there, in a nutshell, is the Histogram. A small but powerful - and useful - panel of Lightroom Classic CC. As always feel free to leave comments or send me a message if you have any questions!