Antimatter is defined as matter which is composed of the antiparticles of the corresponding particles of 'ordinary' matter. For example, an atom of antihydrogen has a nucleus of an antiproton, orbited by a positron. Though in the modern world we can only create miniscule amounts of transient antimatter, the technology has far advanced, allowing for varying applications of antimatter.
Antimatter is created using advanced particle accelerators. These particle accelerators are far more advanced than their modern counterparts, making use of exotic and very expensive technology to produce even small amounts of antimatter. Once the antimatter is created, however, it must be stored safely. Storing antimatter is dangerous due to the fact that it will attract regular matter, colliding and causing a nuclear explosion. Even a single gram of antimatter has enough energy to annihilate a good portion of a city, with a yield of around 50 kilotons. Antimatter is so powerful because the reaction between it and regular matter is entirely efficient unlike conventional fission or fusion reactions, making use of all of the antimatter. To store antimatter, it must be kept within a perfect vacuum and it must be suspended, both of which require certain advanced technologies. The safest way would be to store the antimatter within a large capsule that contains a stasis field.
Antimatter is primarily used in antimatter weaponry, but also saw uses in weak conventional warp drives before erchius fuel was discovered.