Through The Lens flash metering.
TTL flash is basically an improved form of automatic flash control that takes account of the film speed, lens aperture setting, and any filters that might have been fitted to the camera.
As with all automatic features they cannot interpret the artistic wishes of the photographer. For example, you may want to fill in detail on a subject that is partly shaded and this may require you to override settings.
System Flash
It follows, that a TTL flash system requires a TTL capable flash unit and a TTL capable camera; and less obviously, that the two units must be compatible (i.e. they must conform to the same electronic interface standard).
Dedicated Flash - Describes an electronic flash that is made to be used only with a specific model of camera. Canon, Nikon, Olympus and other cameras have specific electrical contacts in the hot shoe to pass TTL-metering and AF range data to/from the flash unit. You can not use a dedicated Canon flash on a Nikon camera for example.
Technical
Although the burst of energy from a flash unit may seem instantaneous, it actually lasts for about a thousandth of a second (~1ms). Reducing the time duration of the flash, i.e. can therefore control the light output, by switching off the current in the tube before the storage capacitor is fully discharged.
A TTL flash unit is one that can accept a signal from the camera telling it to quench.
The important thing to understand about this system is that the flash unit plays no part in calculating the exposure ; it simply accepts the instruction to switch off at a point determined by the electronics in the camera, and will usually issue some kind of error signal if no such instruction is received.
The camera determines the exposure by measuring light reflected from the film and summing (integrating) it over time. It compares the amount of light received against a level determined by the ISO (ASA) setting, and issues a stop ( quench ) signal at the appropriate point. If you reduce the amount of light falling on the film by stopping down the lens or fitting a filter, the camera will take longer to issue the stop signal.
The camera can therefore compensate for all relevant variables until a point is reached when there is insufficient light to complete the exposure; in which case no stop signal is sent, and the flash unit issues a warning