There is more information available on this subject at Antimatter on the English Wikipedia.

Antimatter is the physical counterpart to matter. Antimatter consists of antiparticles which have the same mass as their matter counterparts but with a reversed charge. Antimatter annihilates upon contact with matter, theoretically releasing one hundred percent (100%) of all matter held within the annihilating particles; mainly in the form of Gamma rays (photons) and other matter/antimatter particles. To put this into perspective, the energy released in a hydrogen bomb is only 0.007% of the energy within those particles.

The Covenant use antimatter as a reactant in ordnance, namely antimatter charges. Certain Forerunner weapons use antimatter as well; the Z-390 incineration cannon, for example, fires antimatter embedded within streams of energized particles.