Grounding is a physical connection to the earth, a frame or other external common area to help limit voltage buildup. Grounding protects users from defects in insulation in an electric device, and can also be used to prevent the accumulation of excessive static electricity. Electricity always takes the path of least resistance. If there is "stray current" it will follow along the ground wire and harmlessly flow out of the unit instead of back through the unit and into the person touching the device.
One type of electrical grounding is found in three-pronged appliance plugs. Some people are tempted to cut off or remove the third prong because they have a wall socket which only accepts two-prong inputs. Never remove the third prong--it is the grounding device for the appliance. Without the ground, if there is a short or defective wiring in your appliance you can potentially receive a dangerous electrical shock.