Like any plucked instrument, mandolin notes decay to silence rather than sound out continuously as with a bowed note on a violin, and mandolin notes decay faster than larger stringed instruments like the guitar. This encourages the use of tremolo (<answer>rapid picking of one or more pairs of strings</answer>) to create sustained notes or chords. The mandolin's paired strings facilitate this technique: the plectrum (pick) strikes each of a pair of strings alternately, providing a more full and continuous sound than a single string would.
What is tremolo?
rapid picking of one or more pairs of strings