Tellerium was a necessary material used to stabilize the matter/antimatter reaction in a warp core. On an Template:ShipClass starship, a small vial of tellerium was useful for an extended period of time.
The USS Voyager nearly ran out of tellerium in early 2372. In a desperate attempt to fill a vial of tellerium, Kathryn Janeway, Tuvok, B'Elanna Torres, and Neelix secretly transported to the Alsaurian homeworld and were nearly all captured by the Mokra Order. Neelix made it back to the starship with the tellerium. (VOY: "Resistance")