Drop 2 chords are built by dropping the second highest note of a close chord down an octave, thus giving four supplementary ways of playing a seventh chord (min7, maj7, dom7, m7b5, dim7, aug7, augMaj7).
This free music theory cheat sheet will help you understand how these drop 2 voicings are built. You can easily get it for free (or name a fair price) in PDF format on GUMROAD.

