It adjusts the clutch (automatic)...
too much (decrease) it will drag at idle, be hard to shift etc..., not enough (increase) it will slip and wear out the clutch.....
Edit..
BTW if you didnt know... the Nut is a Lock nut.. (Think Valve adjustment)... just there to keep the adjustment... so you hold the center with a screwdriver, losen the nut with a wrench.. make your adjustment. then reverse the order...