tapping is generally when you use your picking hand to hit a fret and produce a note. normally, after a note is tapped by the picking hand, the note is "pulled-off" to a lower note on the same string, either an open string, or a note that is fretted by the fretting hand.

if you want to learn more, schmange has a whole section on tapping, one or two handed, i think he goes up to eight finger tapping.

i'm sure someone else here has much more to say on this subject and more helpful descriptions of tapping, but i got here first.