Put simply, a light meter is a device that measures the exact amount of brightness or light present in a scene.
A Little More Detail:
Light meters are essential in determining the perfect exposure setting in terms of what shutter speed and aperture combination to use based on the brightness in the scene.
There are two types of light meters.
1. Reflective Light Meter The in-built meter found inside DSLR cameras measures light reflecting off a subject and then calculates an appropriate shutter speed and aperture size for the correct exposure. This is known as TTL (Through-The-Lens) metering. There are different metering modes such as Evaluative, Centre-Weighted, Partial Metering and Spot Metering. Different scenes and lighting conditions call for different metering modes however Evaluative is the most popular metering mode.
2. Incident Light Meter These are usually hand-held devices that measure incident light (light that falls on a subject).
The advantage of using a light meter is the ease of measuring ambient and incident light falling on a subject. Photographers and their assistants can quickly calculate the correct shutter speed and aperture value using a light meter.