A voicing is a kind of chord classification that is somewhat similar to that of an inversion. As a premise we number the chord tones by their appearance from top to bottom as can be seen in the image above.
Very much to the left in the picture above we see a C maj7 chord in root position. The voicing of this configuration is called closed voicing which in essence means that all notes are as close as possible to each other. The closed voicing is very useful for analysis, because it makes it easy to see what inversion we are in. So sometimes when unsure about the exact nature of a chord, reducing it to a closed voicing can be very helpful.
Drop voicings are generated by dropping a specific voice (i.e. 2) of a closed voicing by exactly one octave. In the illustration above we can see how the drop 2, drop 3, and drop 4 voicings are each generated from a closed position voicing. Each of these voicings have a very particular character, which is why we are interested in them.
You may have noticed that we have drop voicings for all but one chord tones. Why don't we have a drop 1 voicing? Well, a drop 1 voicing would be the same as a third inversion.
The reason we differentiate between inversions and voicings has to do with how harmony is perceived by the human ear. When hearing a chord the top note of it makes a much stronger impression on the listener than any of the inner voices. It sticks out. Secondary is the bass note i.e. the lowest note of that chord. The inner voices are not easily identified by the untrained ear, but nevertheless provide most of the coloring a chord has. Their impact remains mostly unconcious. Due to this fact we pay a little more attention to the top note of chords when arranging, composing or simply playing music. Inversions determine the top note of a chord and voicings the distribution of the other notes below it. One is more akin to lines where the other one can be thought of as coloring.
In ChordLab voicings are adjusted using the voicing buttons in the right pane of the main window or the Voicing menu. The latter defines the keyboard shortcuts option-1 through option-8 which can be very useful. Additionally available are the cmd-shift-[ and cmd-shift-] keys to move to the previous or next voicing.
Note: The voicing buttons are simultaneously indicators of the current voicing. As shown above a little black arrow appears in the button for the selected voicing (in this case the drop 3 voicing).