Both Lightroom and Photoshop provide a tone curve function (along with a million sliders) allowing us to fine tune our digital files graphically.
What is the tone curve? Here is a Wikipedia definition:
“In image editing, a curve is a remapping of image tonality, specified as a function from input level to output level, used as a way to emphasize colours or other elements in a picture. Curves can usually be applied to all channels together in an image, or to each channel individually.” https://en.m.wikipedia.org/wiki/Curve_(tonality)
The tone curve is essentially a one line representation of the histogram and the histogram is a map of all the pixels in our photo. We can push around the curve to modify the look of our photos which then changes the histogram.
The easiest way to start is by simply sliding the sliders underneath the curve graph and examining the results.
It is an interesting tool and worth playing with to see what it does.