Pull offs are the opposite of hammer ons. A hammer on will always go up in pitch (you hammer on to notes above your starting point). Pull offs will always take you down in pitch (you pull off to notes behind).
With a hammer on, we play a note, then hammer onto a higher note on the same string. With pull offs we do exactly the opposite. We start with a higher note and pull the finger off the string to sound out a note lower down the string. For this reason, we often have to put our fingers in such a way that when the note is pulled off, you are fretting the next note down the string already. This technique can be awkward at first so make sure to watch this video and try out the following exercises in this section to learn the best way to do them.