Fixed gear shaft: The first shaft is to receive and transmit engine power, namely input shaft. Middle shaft is to transit the power on first shaft and then transmit to the second shaft. The second shaft is to transmit the power to differential mechanism to drive axle, namely output shaft.
Essentially, the gear shaft is the axle of the gear, providing the rotation that allows one gear to engage with and turn another. The process is often known as gear reduction and is essential to transferring horsepower from engine to drive mechanism.