T for TTL Metering
TTL is for Through The Lens. This is a type of auto metering where the camera decides the exposure values and white balance etc based on the incident lights, measured through the lens, hence the name. It is also used for flash metering, where a pre flash is fired and the response acts as the deciding value for the flash power. In a fix, Auto mode works fine, but often we want more control and this setting is left untouched. Do explore it though, might come handy someday.