This is an Exposure Line. An Exposure Line tells you how bright or dark a picture is. If the arrow is above zero the photo would be Over Exposed as it would be too bright, if it was below the zero it would be Under Exposed because it would be too dark, whereas if the Exposer Line is on the zero, the image would be perfect.
Exposure is the amount of light that is in the image.
Under Exposed is where the image is too dark, Over Exposed is where the image is too bright, when the exposure is in the middle it is just right.
This photograph is Over Exposed. Over Exposure is when the focus point on the camera is too bright.
This photograph is Under Exposed. Under Exposure is where the focus point on the camera is too dark.
This photograph is perfectly Exposed. Perfect Exposure is when the focus point on the camera is perfect.