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