A hammer-on is a technique where the first note is picked, but the second note sound is produced by "hammering" the left hand finger onto the fretboard. The hammer-on is indicated on the music staff by a curved line joining two different notes. In Tab notation the hammer-on is indicated by a curved line with the letter H above it.
The hammer-on is performed by playing the first note (in the following example – a G note – open third string) with the thumb. While this note is still sounding, the second finger of the left hand hammers down onto the second note (an A note on the 2nd fret of the third string). The second note is not picked; the sound is produced entirely by the left hand.