Propeller Shaft is the center drive shaft, between your gearbox/tranny and the final drive.
It is connected by 4 or 6 bolts & nuts per coupling at both ends. Forces like torque is transmitted through this intermediate piece and if the joints are loose or misalign, vibration is induced. U can actually hear it (sounds like an uneven worn out tyre).
Therefore Re-tensioning is required to keep the shaft intact. Not too sure whats the reccomended maintenance schedule though.
But my gut feel is every 100,000KM.
Cheers!