Plain English Meaning
It is the part of an attitude indicator that quietly keeps the instrument's internal spinning wheel standing straight up so the artificial horizon stays accurate. If the gyro starts to tilt out of position, this mechanism nudges it back on its own, without the pilot doing anything.
Full Definition
A built-in feature of a gyroscopic attitude indicator that automatically returns the gyro's spin axis to the vertical, keeping the instrument aligned with true vertical during normal operation. In an electric attitude indicator, this is typically accomplished by small torque motors driven by liquid-level or mercury switches; in a vacuum-driven attitude indicator, pendulous vanes hanging from the gyro housing use airflow from the instrument's own supply to apply a corrective whenever the gyro drifts vertical.