Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. An atom of anti-hydrogen, for instance, is composed of negatively charged antiprotons being circled by postitively charged positrons. If a particle/antiparticle pair comes in contact with each other, the two annihilate in a burst of energy.
With antimatter, the entire possible energy of the matter could be harnessed, instead of the very small chemical energies or nuclear energies that can be extracted today. Since the energy density is vastly higher than these other forms, the thrust to weight equation used in rocketry and space craft would be very different. In fact, the energy in a few grams of antimatter is enough to transport a small ship to the moon. A matter, antimatter anihilation reaction turns the entire mass of the matter and antimatter into energy. This is far more effecient than even a nuclear fusion reaction. It is hoped that antimatter would be used as fuel for interplanetary travel or possibly interstellar travel.
As far as we know there are no antimatter atoms in existance in our universe. This is a great mystery since one would expect matter and antimatter to have been generated in equal amounts after the Big Bang. The scarcity of antimatter has given us a stable universe, however, without which life could not have evolved.
The scarcity of antimatter does mean that it is not readily available to be used as fuel. Generating a single atom of antimatter is immensl difficult and requires particle accelerators and vast amounts of energy. No more than a handful of atoms have ever been made.