By definition, the guard terminal is nearly at the same potential as the high impedance terminal, so the guard terminal will be at the same potential as the magnitude of the voltage source.
Figure 2-14 illustrates how a driven guard prevents the leakage resistance of a cable from degrading the low current measurements. In the unguarded circuit of Figure 2-14a, the leakage resistance of the coax cable is in parallel with the DUT (RDUT), creating an unwanted leakage current (IL). This leakage current will degrade very low current measurements.
In the guarded circuit shown in Figure 2-14b, the inside shield of the triax cable is connected to the guard terminal of the SMU instrument. Now this shield is driven by a unity-gain, low impedance amplifier (Guard). The difference in potential between the Force/Output HI terminal and the Guard terminal is nearly 0V, so the leakage current (IL) is eliminated.
A setup axample
Read more from Low level measurement handbook - tth edition, by Keithley.