Through The Lens metering. The basic method for determining photographic exposure is to measure the intensity of the ambient lighting and choose camera settings accordingly. It can apply to both flash and ambient light readings.
The problem with metering is that filters, lens extension, and other factors must be taken into account, and the way around this problem is to examine the light that comes in 'through the lens'.
One obvious advantage of this procedure, is that the light reading so obtained can be used to set-up the camera automatically; but a major disadvantage is that the reading is dependent on the reflectivity of the subject.
TTL metering systems assume that the subject will reflect 18% of the light falling on it. Many subjects do reflect about this much, and are said to be of 'average reflectance'; but many do not and the resulting exposure can be seriously incorrect.