it is used in situations where the tiny leakage of current through the insulating surfaces of a wire or pcb board would otherwise cause error in the measurements or functionality of the device. the basic principle is to shield the sensitive wire by surrounding it with a conductor that is held at the same voltage as the wire. if they are at the same voltage then there will be no current between the sensitive wire and the shield. this is typically achieved using a voltage buffer/follower that matches the guard voltage to the wire voltage. the leakage from the shield to other circuit elements is of little concern as it is being sourced from the buffer which has a much lower output impedance.