Antimatter reactors is a power generation device that utilized antimatter. Presumably they are stronger than Fusion reactors, but more dangerous to operate and require constant monitoring.
During the 25th century, antimatter reactors were the latest thing in shipboard power generation. But they were only being used by the military at the time. In 3472, Aurora was the first new ship to utilize antimatter reactors since the Bio-Digital Plague.
Antimatter containment is achieved through the use of electromagnetic fields. In event of containment failure, you should duck and cover as truly scary antimatter explosion/shockwave is coming your way ( such as the one caused by Jung gunboat overloading their antimatter reactors (EP1, that thrown Aurora into Pentaurus Cluster). REMS-poles are used to remotely manipulate EM field emitters within the outer containment bottle on the antimatter reactors.
Energy from the antimatter reactors is distributed through the power distribution system . Explorer Class ship utilize four antimatter reactors, among other thing they power the propellant acceleration tunnels, which are required for its main engines.