What is a Ground Loop? If your computer is at ground G1 (i.e. G1 = number of volts between earth ground and the metal chasis of the computer), and the instruNet device is at ground G2 (i.e. G2 = number of volts between earth ground and the instruNet GND screw terminal), and the device under test is at ground G3 (i.e. G3 = number of volts between earth ground and the ground pin on DUT); and these 3 grounds vary by several milliVolts (as noted here), then attaching a wire between G2 and G3 (e.g. the common (-) wire within the cable that attaches your sensor to instruNet) will cause current to flow on that wire, and this current will induce a voltage drop along that wire. This is called a "ground loop".
How can I determine if this is causing a problem? If you are working with low level signals (e.g. < 100mV) and the readings are eradic or incorrect, your problem might be related to high frequencies between grounds, induced by a ground loop.