A flash meter is a hand held instrument that is used to measure the amount of light from the flash that will reach the subject you want to photograph. This is called the incident reading. This is helpful because you need to know if your light is adequate enough for a good photograph. This information will help you set the right aperture and shutter speed on the camera or it will let you know how far to set the distance of the flash unit from the subject.
The flash meter can be used to measure either the amount of reflected light from the subject or to give you the incident reading to find out how much light will reach the subject. The incident reading is the quickest and more accurate way to photograph in difficult lighting situations.
More expensive flash meters enable you to plug a flash sync cable into them, which lets you trigger the flash by pushing a button on the end of the cable. Meter cases are also available. Meters range in price from about $50 to $1,000.