In photographic parlance, 'TTL' means 'through-the-lens', and particularly refers to a light meter
which - as you may have guessed - takes its reading through the lens of the camera to which it is integral
The advantages of a light meter which takes its readings through the lens are that, firstly, if filters are connected to the front of the lens, the TTL meter will be able to compensate for any light they block; and secondly, a TTL meter is less liable to be interfered with by spurious light from the sides of the scene.
The alternative to a TTL meter is a Non-TTL meter, a feature common on cameras made before the 1970s, the meter most commonly mounted just above the lens. In the case of a Non-TTL meter, the photographer has to guess what kind of effect filters are having on the exposure; furthermore, the lightmeter tends to draw light from outside the photographic frame, meaning that (for example) a photograph taken indoors, in daytime, through a window to the outdoors will tend to be overexposed. Non-TTL meters are typically selenium cells which power a small indicator, the exposure calculated from a complex scale mounted on the camera's top. If the photographer is clever enough to use it, the calculator can be handy for visualising the effect of different shutter/aperture combinations, and thus non-TTL meters, in the right hands, can have a slight speed advantage.
Nowadays, of course, the vast majority of all types of cameras are capable of determining the correct exposure and setting the photographic variables accordingly. With the exception of studio flash photography, lightmeters in general are now the preserve of only the most committed photographers.