A light meter is an external device that you can actually place on your subject to read the light where the subject is. The meter will tell you what your aperture, shutter speed, and ISO need to be set at. Or, you can tell your light meter what ISO you're at, and then it will tell you what your shutter speed and aperture need to be. It's an easy way to tell the camera exactly what settings to be on because sometimes the internal sensors on digital and film cameras aren't 100% accurate.