A chord often has four different intervals. On guitar, the closest chord voicing we can create is on four adjacent strings. Drop voicing is a method for “widening” out these intervals, by moving one or two of them to a different octave, creating an inversion (unless the root itself is dropped).

The highest pitch in such a chord is known as the first (or top) voice. The lowest pitch is the fourth (or bass) voice.

The rearrangements we can use are:

• drop 2? (drop the second voice)
• drop 3 (drop the third voice)
• drop 2,4? (drop the second and fourth voice)

This greatly expands your available chord vocabulary

This video demonstrates this.