What are and how do use exposure meters?

I have a Nikon D3100 and I don't know how to use exposure meters. Please help explain what they are and how to use them? My manual doesn't say much about them.

You don't need an exposure meter. Just use your camera's internal light meter.

All in camera meters are REFLECTANCE meters. They read light reflected off of the subject / scene. All such meters seek to expose at what is known as a tonal value of middle 18% grey. This works ok in most cases, but many times it will not, especially if your subject if very light or very dark. Look up the Ansel Adams Zone System to get a better understanding of what is happening with an in camera meter. To get good, consistent exposures with the in camera meter, you need to learn how to use exposure compensation to correct for readings that are not going to be accurate.

An external, hand held light meter is an AMBIENT meter. It reads light falling ON a subject / scene. You put the meter in the light that is on your subject and aim the meter back to where the camera will be. You then simply push a button and the meter will give you a much more accurate exposure. It will display this as a correct shutter speed and aperture. You can usually also scroll the meter to get various combinations of shutter speeds and apertures that will all give the same exposure. This allows you to use a particular shutter speed or aperture depending on what you are wanting or needing to do with the photo.

I use an external meter as much as possible. It is simple and fast and gives more accurate exposure. As an alternative, you can use what is known as a GREY CARD in the scene. You move your camera so that the grey card fills the frame, (or use the spot meter function of the camera). As this card is the correct tonal value for which the camera seeks to base exposure, you can use the readings the camera gives you from the grey card and have very accurate exposures.