You can see this when looking through your viewfinder, and on some cameras you can also see it on your LCD screen. When the tick mark (see red arrow) is lined up in the middle, your camera is telling you it is a good exposure. If these tick marks line up towards the positive numbers, your image will be too bright (overexposed). If they line up below the negative numbers, your image will be too dark (underexposed).
In these images below, you will see the light meter through my viewfinder. This image would be too bright. My ISO number is pretty high, letting in a lot of light. Think (+) as too much light.