Pull-offs are a very common guitar technique, something you will want to get happening as soon as you can. They're mostly used when playing melodically, but can also be used when playing chords, especially the open shapes. A 'pull-off' is the opposite of a hammer-on. Where hammering on allows you to go up in pitch without picking the new note, pulling off allows you to go down in pitch. That's because when you pull a finger off a string to another fret, it can only be toward the nut, which lengthens the string, and the pitch must go down.
Of course, you must already be holding down the next note(s). The action is a kind of sideways pulling twang and release from the finger holding down the first note, so you're actually plucking the string with the fretting finger. When enough twang is applied, the string vibrates as the new (already fretted) note. The movie above shows a bunch of pull-offs. These hammer-on/pull-offs become second nature very quickly with a little practice, and really come in handy. The term 'legato' is used to describe this way of playing.