There are many different kinds of light meters, but generally there are a couple of readings that they will allow you to take. Your camera has a reflective meter, meaning that it measures the light reflecting off the subject. Most hand held meters will also measure incident light which is the light falling on the meter's sensor. Incident light isn't affected by the material of the subject. Many of them can also trigger strobes so that you can measure the light they are producing. The Sekonic L358 is a very popular model and is about $250. This is what I use, because you can buy a chip to put in it that will wirelessy trigger Pocket Wizards... avoiding the need to move your sync cable from each strobe to the meter. A meter is a very handy tool for general photography, but absolutely essential when using studio strobes.