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, annihalating both particles. This powers a warp drive, which allows the ship to travel at faster-than-light speeds.
Because of the volatility of any antimatter, including antideuterium, it is kept in magnetic containment pods aboard a starship.
Additional References
- TNG: "Night Terrors"