The High Drive was a sub-light propulsion system fueled by a matter/antimatter conversion process, commonly used by human starships.
High Drive nozzles were mounted on the hull of the ship. They were far more efficient than plasma motors, but to switch to High Drive too early was considered "courting disaster".1:1 During high level use of the drive, a certain amount of antimatter inevitably escaped the conversion process and was voided in the exhaust. When this occured within an atmosphere, matter and antimatter destroyed each other in a destructive energy burst.
Missiles commonly had only High Drive for propulsion.1:3