The machine's crew can move their machine, perhaps to gain a better position to fire. The machine and its crew are moved like a unit of skirmishers, except that they cannot charge enemy units (not even if Frenzied!), can never march and pay normal penalties for moving through difficult ground.
The Movement rate is the speed at which the machine can move with its full crew and it is the same as the crew's - if any crew members are slain, then its speed is reduced proportionally. For example, if the machine is normally moved at normal speed by a crew of three, it will lose a third of its movement for each model below three that is moving the machine.