Stolen from a bloke on PH:
Clutch judder is usually something like oil or some other crap somehow getting on the surface of the clutch itself and causing it to not grip properly or smoothly when engaging causing the juddering effect as it grips in part, then slips, then grips and so on.
It can also be caused by other parts of the clutch or flywheel being slightly out of shape usually from being just plain worn out or glazed to the point it's not getting consistent grip.
It's usually a sign your clutch/flywheel might need to be looked at/replaced at some point in the near future. If it's only minor or infrequent then it will likely not need urgent attention. In a newer car, or a make of car with a known problem in that area I would get it looked at sooner in case there is some underlying problem that is causing it.