That's the histogram
Basically you use it to help expose your photo's correctly
Depending upon the type of photo and lighting. You try to keep the histogram from touching the left or right sides.
The far right is usually over exposed and lose details in the white, far left is losing details in the dark, under exposed
You try to keep it around the central area. But again this varies with the type and style of your photos
Sometimes crushing the blacks adds to a scene and same with over exposing
It's just a guide to help you get the right balance for your composition
