The incident meter is something of the opposite of the reflective meter. It measures the light falling on the subject, rather than that being reflected from the subject. These are the most common type of handheld meter.
The incident meter uses a diffusing dome over its sensor, which is pointed at the camera from the subject. The ASA (film speed) is set, then the meter is activated. Depending on whether the meter is analog or digital, you either dial in the given exposure value, or with digital, the meter will display an exposure setting.