A hammer-on is a playing technique performed on a stringed instrument  

 (especially on a fretted string instrument, such as a guitar) by sharply bringing a fretting-hand finger down on to the fingerboard behind a fret, causing a note to sound. This technique is the opposite of the pull-off. 

 Passages in which a large proportion of the notes are performed as #hammerons and #pulloffs instead of being plucked or picked in the usual fashion are known in classical terminology as legato phrases. 

The sound is smoother and more connected than in a normally picked phrase, due to the absence of the necessity to synchronize the plucking of one hand with the fingering on the fingerboard with the other hand; however, the resulting sounds are not as brightly audible, precisely due to the absence of the plucking of the string, the vibration of the string from an earlier plucking dying off. #hammeron