Gaurang:
A variac is essentially a power transformer that is designed in a way that the turns ratio of the primary to secondary ic controlled by turning a large knob or moving a lever. The internal connections in these things is often wired up as an auto-transformer wherein the secondary winding is actually part of the primary winding.

To make one of these things servo controlled means to have a mechanical servo positioner able to move the lever or turn the knob.

Michael Karas