A close relative of the bulb (B) shutter speed setting and, like bulb mode, it’s used for long exposures. With time (T) exposures, though, you don’t hold the shutter button down all the time – you press once to start the exposure and a second time to end it.
Usually, the camera’s exposure time is set by the shutter speed you’ve selected, so that the exposure ends automatically. But in Bulb mode the shutter stays open for as long as the shutter button is held down, so it’s used a lot for night photography, where exposures can range from 30 seconds to 30 minutes (in moonlight). In the old days you’d use a cable release with a locking screw; these days you’d use a remote release with a bulb mode (using a remote release means you don’t risk moving the camera by keeping your finger on the button).