Watch the Fretboard Tapping online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
Fretboard tapping AKA hammer-on-from-nowhere or ghost hammer is a vital part of playing tapping sequences across multiple strings. It's simply a fret hand finger fretting notes on the neck sans pick attack hence the nickname "hammer-on-from-nowhere". Think of it as advanced guitar connective tissue – totally essential.
Looking for inspiration? Anyone who's worth their tapping salt employs this utilitarian tapping technique. Without it, tapping sequences across multiple strings, such as the example from Van Halen's "Jump" I play in the video, would be way less cooler.