0 is the perfect exposure for a picture but -2 is under exposure
this picture is under exposure which makes it look very dark.
this picture is over exposure so it look very bright.
this picture is perfect exposure whih mean it is the right brightness.
exposure is how much light you let into the camera so if you leave your camera out for to long more light will get into the picture which will mean it will be overexposed this mean that the image has too much light and will look very white but if you leave the camera for a short time then it will not have as much light that is needed so the image will be underexposed so the photo will look very dark.