Bah...you've got the website right here

Do a search for 'tapping' in the technique forum and I GUARNATEE you'll have enough reading to keep you busy for weeks.
To sort of quickly answer your question, tapping is really just another way of striking/fretting notes. You use your left hand to 'hammer' notes, instead of fretting and plucking. Later, you can use your right hand to do exactly the same thing; that's called two hand tapping.
But trust me, do a search...you'll learn a lot. Take care.
Alex