Antideuterium is an antimatter isotope of hydrogen which contains one antiproton and one antineutron in its nucleus, and one positron orbiting the nucleus (just as its matter opposite, deuterium, is made up of a proton, neutron, and an electron).
In a matter-antimatter reactor, such as those used in modern warp-capable starships, antimatter, such as antideuterium, reacts with normal matter. The interaction annihilates both particles and converts their mass wholly into enormous quantities of energy and a coctail of highly energetic sub-atomic particles, which is collectively channeled by a warp drive as warp plasma. This in turn powers a warp drive, allowing the ship to travel at faster-than-light speeds.
Because of the volatility of any antimatter (it will react with violently with ANY matter it comes into contact with, such as that which composes a standard containers walls), it is stored in specialized antimatter pods, which were surrounded by a magnetic containment field aboard a starship per SFRA 3287.0.