Guard voltage is applied to the shield of cable to cancel any capacitance between sense electrode and environment except in the sensor.
For high sensitivity applications a dual-concentric coax cable is used - the outer shield is grounded, the inner shield is fed the guard voltage and
thus the inner conductor is both guarded and the guard voltage isn't allowed to affect the environmental fields. Used for EEG/ECG etc from what I recall.
The CapSense library is nothing to do with all this, which is for analog voltage probes.