TFT, or thin film transistors, are a specialized type of field-effect transistor, which modify the normal design of a FET device.  TFTs are created by depositing a channel material onto an insulator, which in most cases is a form of glass.  The material deposited is usually silicon, which is normally the base structure that non TFT FET devices are built upon.  On this thin silicon deposition layer the rest of the transistor is built, including the dielectric compounds and source and drain contacts.

For TFT display modules, there is one transistor associated with each pixel.  Since the overall size of the TFT transistor is very small compared to the overall size of the pixel it controls, it blocks very little of the backlight which shines through the individual pixels of the display.  The only way this is accomplished is because the thin film of substrate silicon is cleared away from all non-transistor areas (most of the area of the display) so that light has an unobstructed path through the glass transistor base.  If the substrate had been actual silicon, it would not have permitted light to pass through and would lead to a nonfunctional display.