Open voicings are formed by dropping one of the voices of a close voicing down one octave. In jazz there are 3 common open voicings (Voices are counted from the top):

drop 2 - dropping the second voice down one octave drop 3 - dropping the second voice down one octave drop 2 and 4 - dropping both the second and fourth voices down one octave

4 way close  Play (help·info)

Drop 2  Play (help·info)

Drop 3  Play (help·info)

Drop 2 and 4  Play (help·info)

In actual practice it is not necessary to adhere to just one of these voicings in harmonizing a melodic line. Any of these open voicings (and closed) may be freely intermingled in order to achieve smooth and interesting inner voice motion.