A hammer on allows you to play a note by "hammering" a fretting-hand finger onto the string hard enough to make the note sound out without plucking or strumming the string.
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Hammer ons give a smoother, more legato sound compared to picking notes individually. They’re often performed when one finger frets a note on the fretboard and an additional finger on the same string hammers on, resulting in a brief sound of the previous note before the hammered-on note sounds.