The guitar techniques Hammer-ons and Pull-offs are used in the main riff in this song. The terms hammer-on and pull-off were invented and popularized by Pete Seeger in his book How to Play the 5-String Banjo.
Pull-offs: Bar the strings on the 7th fret with your 1st finger on your left hand and hold down the 3rd string on the 9th fret with your 3rd finger on your left hand, and pick the note as usual with your thumb on your right hand. Then pluck the 3rd string with the 3rd finger on your left hand at 9th fret on the fretboard, so the 3rd string now vibrates on the 7th fret. Repeat this pattern on the 4th string twice.
Hammer-ons: Bar the strings on the 7th fret with your 1st finger on your left hand and then pick the note as usual with your thumb on your right hand. Then bring down your 2nd finger on your left hand on the 8th fret on the 3rd string, the quite opposite of the pull-off.
Chorus: The first 4 strumming chords are played on the 7th fret: B, (bar 7), Ama9/B and (bar 7). The next 4 strumming chords in the chorus are played on the 5th fret: A, (bar 5), Gma9/A and (bar 5).