What is it?
It's a basic left-hand tecnhique which is used to add color and variety to single-note playing. Also it will gives more speed to your performance.
How to play it
It is produced by playing one note with the pick and then playing a second note by striking the fretboard with a finger of the left-hand. These notes are played on the same string. It's indicated by a "H".
Below is an example of hammer-ons on two strings.
And below the E minor pentatonic scale played with hammer-ons.
Play now these hammer-ons on the A minor pentatonic scale.
And below two easy nice guitar licks with hammer-ons.