Antimatter is a form of matter composed of particles with the same mass, but opposite quantum properties (e.g. charge) to those of corresponding particles of ordinary matter. This theme is used whenever a property of antimatter is explicitly touched on in the story.

Notes:
The typical use case occurs when somebody explains that matter and antimatter go boom when brought into contact. In more scientifically accurate treatments of the subject, however, we may learn that a collision between a particle and its anti-particle partner leads to their mutual annihilation, giving rise to a release of energy in the form of radiation.

Examples:
The danger of trying to mix matter with antimatter was explained in tos1x20 "The Alternative Factor".

References:
https://en.wikipedia.org/wiki/Antimatter