A torpedo tube was a compact area of a starship where torpedoes were 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 or armory. (ENT: "Silent Enemy", ENT: "Fight or Flight")
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")
In 1996, Captain Kathryn Janeway entered one of the torpedo tubes of the USS Voyager (which was temporally displaced from the year 2373) in order to bypass the damaged launch activation sequencers for a manual launch. Ensign Harry Kim warned her that without the activation sequencers, it would be hazardous for her to be in the torpedo tube due to risk of plasma exhaust ignition. Janeway nevertheless manually fired the photon torpedo from inside the tube in order to stop Henry Starling from causing a temporal explosion that threatened to destroy the Sol system in the 29th century. Commander Chakotay, knowing the risk to Janeway, ordered The Doctor to go down to the torpedo tube to render medical attention. The Doctor however, was unfamiliar with the internal layout of Voyager, having previously been confined to the sickbay and was accompanied to the torpedo tube by Ensign Kaplan. Janeway suffered minor burns as a result. (VOY: "Future's End, Part II")