When antimatter comes into contact with matter it annihilates: the mass of the particle and its antiparticle are converted into pure energy.
Unfortunately, however, antimatter cannot be used as an energy source. Although the annihilation of matter and antimatter releases energy, antimatter does not occur in nature: it has to be created. This requires in itself a lot of energy. Even the storage of antimatter requires a lot of energy.
The inefficiency of antimatter production is enormous: you get only a tenth of a billion (10-10) of the invested energy back. If we could assemble all the antimatter we've ever made at CERN and annihilate it with matter, we would have only enough energy to light a single electric light bulb for a few minutes.