Grounding is, plainly and simply, reestablishing a normal heart rate and
metabolism after working magic. IF you are working magic correctly, your
unconscious mind and conscious mind (or to use the Jungian model, id and
ego) should be functioning equally (the conscious mind directs the
unconscious to create a result). In normal waking life, the unconscious is,
well, unconscious. So grounding is leaving the altered state and returning
to the normal metabolism, heart rate, and awareness of everyday life.          
Some people do this with visualization (as with Starhawk's tree meditation).  
Gareth Knight suggests eating a grounding meal (heavy foods, like meat or     
pasta) after a ritual or magical working. I find turning on a room light and  
playing some loud harsh music can snap me right out of a trance.