The pull off is a technique often used in conjunction with the hammer on. It involves the finger flicking the string as it pulls away from it, creating the sound of the note.
The pull off is indicated by the curved line, and the small "P".
Here is an example, using the G chord:
In this example the hammer on and pull off are played on the 3rd beat. This involves the use of triplet notes (indicated by a curved line and a 3), which have the same timing as that of the triplet rhythm, introduced in Lesson 17.
Remember to actually pull the string as you pull your finger away, as opposed to just lifting the finger off the string. Try the following example, involving hammer ons, pull offs and triplet notes.