It is to adjust the thrust bearing mechanism. It pushes against the shaft so assuming it adjusts the play in the clutch pedal.
I have undone mine all the way and then screwed it in so it meets the shaft. Once my gearbox is back in I will readjust along with the clutch cable.
Here is a picture of it on the inside (handy I have mine on the bench at the moment)
[attachment=0:2makrsdo]P1070721.jpg[/attachment:2makrsdo]
Anonymous