When you check the histogram, it gives you an idea of how your image is going to look. Think about the mood you want to capture and try not to rely on the LCD preview.
The main reason you want to check the histogram is to avoid clipping. You do not want to lose details in the shadows or highlights of your image so make sure your histogram does not have spikes pressed against the edges of the graph.
If the graph presses against the left side your image is underexposed.
If the graph presses against the right side your image is overexposed.
If the graph has gaps on both sides your image has little contrast.
If the graph presses against both sides your image has too much contrast.
Another way to preview highlight clipping on your image is to turn on the “highlight warning” on your camera. This will make the lost highlight details blink on your LCD.
To get the best exposure checking the histogram, here is a trick that many people do not know:
Do you see those faint vertical lines on the histogram? Each line represents a “stop”, so you can change your settings accordingly. In the example below, you can still underexpose or overexpose by half a stop before clipping your image. Try to have the maximum coverage from left to right on your histogram, do not worry about the shape.