Antimatter is the opposite of matter. More specifically, it's a substance formed of antiparticles: positrons (positively charged electrons,) antiprotons (negatively charged protons,) and antineutrons. Bringing matter and antimatter into conjunction causes them to annihilate. Annihilation is the process wherein the reacting matter and antimatter are efficiently reduced to energy—or, in layman's terms, a gigantic explosion. While superficially different from your conventional explosion (being a massive release of gamma-ray photons as opposed to a simple combustion process, or concussive detonation), the upshot is the same, i.e. burning and dying. Antimatter is generated by, essentially, hitting subatomic particles with other particles, such as electrons, at very, very, very high speed. This happens on a very small scale as radioactive materials decay, and is how some types of internal medical scanners work.
Antimatter appears in science fiction continua such as Star Trek, Doctor Who, etc., etc. These canons often use fictionalized forms of it that ignore its extreme incompatibility with normal matter. If the PPC uses antimatter, they're keeping it very quiet. This is probably for the best.
If you are a physicist and this article's brevity and vagueness irritates you, please feel free to correct it. If you are curious about the matter of antimatter and are not a physicist, we suggest you Google it.