Watch the Pull-off Technique online guitar lesson by Cathy Fink from Clawhammer Banjo: Beginner Module 1
The pull-off also helps you play 2 notes on the left hand with only one right hand strike. It works just like it sounds, you have a left hand finger on a note. The right hand strikes the string or strings and then the left hand finger pulls it's way off of the note to another note.
It's easier to watch how it's done than to explain.