ahhh good old black point. Laymans terms
Histogram shows you the data in your picture (looks like a bell curve well should at least look like a bell curve)
there are 3 parts to this histogram.
Black point
Neutral
White point.
When processing your image you want to stretch you data (aka give it more oomph)
when adjusting the Black point (Left hand side) when you move the slider or select a number it makes all pixels under that value pure black. this can be good and bad good because it cleans up your image slightly bad because faint detail can be erased instantly
Neutral (Middle), virtually the same just plays with a different part of the histogram
White point (Right hand side), same as black point but everything above the slider value will be pure white. good for lightening up an image but can erase faint detail.
If you have photo shop you can look at your histogram, but better still look at it in expanded view aka L R G B fiddle with the curves and you will see what happens to the image and also the histograms.!
hope that kinda cleared it up.