Drop 2 voicings are formed by taking a chord and then dropping the next to the highest note, or voice, to the lowest note of the chord. Similarly, there are drop 1, drop 3, drop 2 & 4, etc. Drop 2 voicings are important because most of these chords are easy for a guitarist to play on 4 adjacent strings.
Drop 2 is just a name for how the chords were derived from another chord. This is only important for naming the voicings. We will describe how they are derived then we will show you the voicings.
the complete article here: DROP 2 VOICINGS
The example below shows four C Major 7 chords and their drop 2 voicings in standard notation. The important thing to realize is which note is dropped to the lowest note