Plain English Meaning
A pump driven by the engine pulls air through certain flight instruments. That moving air spins small wheels inside them so they work properly. Without this airflow, those instruments cannot give accurate readings.
Full Definition
An aircraft instrument power source that uses an engine-driven pump to create suction, drawing filtered air across the rotors of gyroscopic instruments such as the attitude indicator and heading indicator to spin them at operating speed. The system typically includes the pump, a relief valve to regulate suction, filters, suction gauge, and connecting plumbing to the instrument cases.