Variable Stator Vanes... Job description
Variable Stator Vanes are generally used in the high stage compressor sections of Turbine Engines. Physical description would be that of a typical stator vane (or Stationary Vane) that is designed to rotate along its longitudinal axis and is controlled by any number of engine control units and typically pneumatically or hydraulically actuated. The purpose is to control the compressed air's angle of attack on the next compressor blade (rotor stage). This both increases the engines efficiency for a given speed/load and prevents compressor stalls via the ability to control blade/stage loading.
At least that is the way it's used on the aircraft I'm familiar with.
Cosmo