A torpedo tube was a compact area of a starship where torpedoes are stored immediately prior to launch. It is a part of the torpedo launcher. Tubes can be controlled from the bridge, or from the torpedo bay. (ENT: "Silent Enemy")
Typically, the area is only slightly larger than the torpedo itself. It lay between the torpedo bay and the torpedo tube doors on the outer hull of the starship. When torpedoes are fired, the outer doors are opened, allowing for the weapon's egress. Tubes may also have inner doors, which protected the torpedo bay from the missile's propulsion system.
Starships often have multiple tubes, angled in different directions away from the body of the ship. Thus, some ships had forward and aft tubes. Some tubes can hold several torpedoes at the same time for rapid fire, or simultaneous launches. (TNG: "The Arsenal of Freedom")