Tapping is basically like having an extra left hand finger to help you perform hammer-ons and pull-offs. Instead of an extra finger, you use either your pick or a right hand finger. If you are performing a three-note pull-off, and you want two notes low on the fret board and one night high, you cannot reach. This is where the tapping comes in. You do the two-note pull off low on the fret board and you use your pick to do the high one. You do this by hammering-on and pullling-off your pick with pressure on the high note and then performing the other pull-off with your left hand fingers afterwards. If you can do this quickly, you can do some really fast solos. Here is an example...

e|--t14p10p7--t14p10p7--t14p10p7--t14p10p7--| B|------------------------------------------| G|------------------------------------------| D|------------------------------------------| A|------------------------------------------| E|------------------------------------------|

Listen

The 't' means tap on the 14th fret with your pick and then pull off onto the 10th and then onto the 7th. This is repeated 4 times. The example below is an example which you should try to get faster and faster until you can do it perfectly! Repeat it over and over until you can...

e|--t14p10p7--t15p10p7--t17p10p7--t15p10p7--|
B|------------------------------------------|
G|------------------------------------------|
D|------------------------------------------|
A|------------------------------------------|
E|------------------------------------------|

Listen

You can use your 1st (index) or 2nd (middle) finger rather than the pick, however it will sound more muffled and not as sharp. If you do want to use your fingers, use your 2nd (middle) finger and hold the pick between your 1st (index) finger and your thumb.