A vernier thruster is a thruster used on a spacecraft for attitude control. It may be a smaller thrust motor than main attitude control motors, or may be simply a small thruster of the reaction control system relative to the main propulsion motors. It is used for fine adjustments to the attitude or velocity of a spacecraft.
Vernier thrusters are used when spacecraft control requirements, such as maneuvering during docking with other spacecraft, dictate a wide range of different thrust levels for attitude or velocity control.
On space vehicles with two sizes of attitude control thrusters, the main ACS (Attitude Control System) thrusters are used for larger movements, while the verniers are reserved for smaller adjustments. As an alternative, larger thrusters which can fire for very short durations or pulses may be fitted instead. Thus, by having a large pulse for a short amount of time, the same vernier end result is reached as when pulsing a much smaller jet for a longer period of time.
Read more about Vernier Thruster: Vernier Thruster Implementations