Chord voicing refers to the way notes of a chord are arranged. The most basic voicing of a chord is in it's triad form in root position; root of the chord as the lowest note in the chord, then the 3rd of the chord, then the 5th. Any chord can be inverted, so the 1st inversion chord voicing would find the 3rd as the lowest note in the chord, then the 5th, followed by the root on top -- one octave higher. The 2nd inversion of the chord would find the 5th as the lowest note, followed by the root, with the 3rd on top -- one octave higher.
Rootless chord voicing involves leaving out the root of the chord but using other intervals and implying the root. For example, you might voice the C major chord with the 3rd and 5th along with some color tones, such as a 6th or 7th, but leave the root note (C) out. But since the 3rd, 5th and 7th of the chord is being used, it implies a root -- C.
Rootless chord voicings from chordman on Vimeo.