The light meter measures this and converts it into f-stops and shutter speeds. The only key to intensity is making sure that there is enough to make an image on the film. OR it is reflected light that has come from light that has bounced off of people or things. This reflected light is called ambient light. The light meter measures the light that it sees and must convert it into a proper exposure.
Most cameras use an averge of all of the light that falls on the frame of the camera.
This quantity of light is converted into aperture and shutter speeds based on the ASA of the film and the amount of light needed to convert that light into a neutral gray on the film. Neutral gray represents a shade of gray that is half way between white and black. Sometimes the average type light meter makes a mistake if it is aimed at a scene that has a bright spot in the frame that throws the average higher or a dark spot that throws the average lower. A snow scene or a night shot or concert shot will fool a light meter. To cure this some photographers use a spot meter which mesures only a small portion of a scene or frame.