A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph. (Wikipedia)
You may have noticed that on our recommended gear page, there isn't anything about which light meter you should buy. So you might wonder, which one do we recommend? The answer: none! Why? Your camera already has a light meter built in. (Take a look at this video on YouTube if you don't know what a hand-held light meter is or how it works.)
Using a separate device will take one more step to check the metering: you'll have to walk back and forth from the camera to the client, changing your camera settings and then metering again. When you just use the light meter in your camera you can simply stand behind the camera, change the settings and then look at the LCD screen to see what the picture looks like. You will need to do the light testing whether you use your camera's meter or a separate one, so the question becomes, Do you want to get the reading from your testing on your camera's LCD screen or do you want it to be on a different device? Sekonic is one that a lot of people use, but Jim and Dustin both recommend NOT buying and using a separate light meter.
So you might ask, Then why do they even sell light meters? If you were a photographer from back in the days of film, you couldn't see what was going to happen in your photo once you hit the shutter button. Therefore, it was very necessary to have a light meter to give you information about your picture. But now that we have such advanced digital technology it is simple to just look at the back of the camera and you get all the information you need. Additionally, remember that when you are taking portraits, you don't want to waste your client's time metering the light (or anything else!). Minimize the fiddling and twiddling you do with your camera while you are in the presence of your client. If you are always fiddling and changing things, your clients lose patience (especially little kids) and then they will have a bad experience which translates to bad business for you. Know your gear, whatever it is, to minimize the amount of time you spend worrying about that in front of your client.