Plain English Meaning
A shop officially approved by the FAA to work on aircraft instruments. If an altimeter or static system needs testing or repair, this is the kind of place legally allowed to do it and sign it off.
Full Definition
A maintenance facility holding an FAA repair station certificate with the appropriate ratings to test, inspect, calibrate, and repair aircraft instruments such as altimeters, airspeed indicators, and static systems. Only personnel at such a station, or other specifically authorized technicians, may perform and approve the periodic tests and required by the for these instruments.