The Histogram shows the total tonal distribution in the image. It's a barchart of the count of pixels of every tone of gray that occurs in the image. It helps us analyze, and more importantly, correct the contrast of the image.
Gray? Yes, photographers know that color prints must first have the exposure and contrast set right, exactly the same as for B&W, and only then do you worry about the color balance.
Technically, the histogram maps Luminance, which is defined from the way the human eye perceives the brightness of different colors. For example, our eyes are most sensitive to green, we see green as being brighter than we see blue. Luminance weighs the effect of this to indicate the actual perceived brightness of the image pixels due to the color components. The world won't end if you simply think of luminance as brightness, that's actually quite fine for our purpose (and it's really fun to watch the purists have a fit anyway <grin>). But luminance is weighted by color, and there is more detail and explanation about luminance in histograms if you're still curious. For now, luminance can be the "apparent brightness" of the RGB pixel tones in the image.
Scanning B&W photos uses exactly the same procedure as described here (for GrayScale, but not for Line art). The histogram shows the tone values as gray anyway.
Every pixel in the Color or Gray image computes to a Luminance value between 0 and 255. The Histogram graphs the pixel count of every possible value of Luminance, or brightness if it helps to think of it that way. Luminance is brightness the same way the human eye sees it, as opposed to absolute brightness. Anyway, the total tonal range of a pixel's 8 bit tone value is 0..255, where 0 is the blackest black at the left end, and 255 is the whitest white at the right end. The height of each vertical bar in the histogram simply shows how many image pixels have luminance value of 0, and how many pixels have luminance value 1, and 2, and 3, etc, all the way to 255 at the right end.
The histogram barchart shows at a glance the relative image tone distribution over the entire range. What it is now, and what it needs. In this image, we have a very high count of pixels that are near, but not at, the white end. We also have many that are near, but not at, the black end. Our image does not totally fill the possible range from darkest to lightest tones. Our image could have more contrast. But we can fix that very easily.
OK, let's begin with the Simple Way technique now...
Umax VistaScan automatically starts each preview at defaults, but Microtek ScanWizard has manual Reset button because it remembers previous settings. There are pros and cons for either way (a configurable choice would be great!). There may be a reason to retain settings for several scans, but ordinarily when starting a new image preview, we want to eliminate all of the adjustments made to the previous image. The Reset button is in every ScanWizard tool, and is also at the bottom of the extended Settings Window (I'd like to see it on the toolbar with the Preview button). It resets all previous adjustments to defaults, for a fresh start for this image, the proverbial known starting place so we know where we are.
Reset, then do a Preview scan. The Preview scan makes the histogram data available. The histogram represents the area of the image that is marked in the Preview area to be scanned. The scanner software stores the histogram data for the entire preview area, but shows in the histogram only the data to represent the currently marked Preview area specified for the scan.
Bar chart of Number of Pixels with that tone goes up (scaled to always nearly reach top)
This is the histogram for this photo seen after the Preview scan. We can just about explain the three peaks in the histogram. There is the highest peak due to the light background towards the 255 end, meaning that many pixels are nearly white. Yellow is way up there too. Then the midrange red and green values, and the very dark green leaves near 0. The peaks mean that the pixel count with those tone values were high. The photo's upper left corner is more white than the other corners, meaning those values are closest to 255. Place the mouse over a part of your ScanWizard graph, and it will show the histogram Gray value there and shows the total count of pixels with that value.
The tone values at the ends with zero pixel count are of the most interest. Note that for this image, the original histogram values don't extend full range 0..255. 0 is the blackest black, and 255 is the whitest white, and we don't have any of either, at either end. But we can expand what tones we do have for best effect, by "Setting the Points". This is a very standard scanner technique, really THE standard technique, and is the reason that any better scanner software provides the Histogram tool. You can ignore it, but you and your scans would be missing out on a really great thing.