Light Meters can measure the amount of light falling on a subject (incident light), or being reflected by a subject (reflective light). By converting these measurements, it defines what would be the most beneficial shutter speed and f/stop to use for that given subject.
Light Meters are particularly helpful where subject matter / lighting conditions are difficult. Some light meters are also able to measure flash light.
For film cameras with a built-in exposure meter, they typically measure an average of the overall scene being photographed. In general, this form of metering will give good results. However, when shooting complicated lighting or complicated subject matter, for example white snow scenes or scenes with high contrast areas, due to the cameras metering taking an average it can lead to an automatic adjustment which could result in over or under compensation of exposure.
A Light Meter can help to overcome this and typically works in two ways:
Reflective readings on a Light Meter will give you accurate results in most circumstances and closely resemble a camera’s meter. They measure light reflected from the subject. This means if the meter sensor is pointed at the snow or dark areas, then similar exposure problems will arise as they would for a camera with an inbuilt meter.
Over or under adjustment compensation can be overcome by taking an incident light reading. The meter measures the light falling onto the subject and therefore does not become confused by the subject's reflective qualities. As it is not trying to achieve an average measurement this type of metering can help avoid under/over compensation problems.