The histogram is a graph that allows you to check the brightness of an image.You can see it in your digital camera while shooting photos or while editing your photos with photoshop,bridge, or lightroom. It is very important to control it while shooting because you will get a better image and will be easier to post processes your shoot.It is very important to start with the better photo possible,if you want the results to be the best.
The greater the bias toward the left in the graph,the darker the image
The greater the bias toward the right,the brighter
In a balanced image the histogram will be like this,
not a lot toward left or right