Quartal voicings stack perfect 4ths (5 semitones each) instead of the usual 3rds. The result is an open, ambiguous sound — you often can't tell the chord quality just by hearing it. This ambiguity is the point: quartal voicings give modal jazz its characteristic floating quality.
Stack four perfect 4ths from the root: R, 4th, b7th, b3rd+octave. Second variation starts from the 5th. Each note is 5 semitones above the previous.
Over modal tunes that sit on one chord for many bars (So What, Impressions, Maiden Voyage). The ambiguity lets the soloist imply different harmonies over your voicing. Also effective over minor 7th, sus, and minor 11th chords. This is the McCoy Tyner / Herbie Hancock sound.