A close voicing is the simplest way that you can play a three note or four note chord keeping the intervalic separation of the notes as minimal as possible. For example, the close voicing for a C major chord would be noted C E G. The close voicing for a C major 7 chord would be noted C E G B. The close voicing for a B minor 7b5 would be noted B D F A.
Notice that in order for the voicing to be considered a close voicing, the notes have to be in an intervalic consecutive order. For example C G Ewouldn't be a close voicing anymore because in order for you to have G before E, it means that you are now raising the E up an octave.
Now, if you play the piano, then you know that close voicings are also the most basic way to play a chord. Playing the ROOT, 3rd, and 5th straight in a row as close as possible helps beginners visualize the chord, the invtervalic distance, and once you see it, it is easier to move it around different keys.
However if you play guitar, you will notice that playing a close voicing is very difficult because in order for you to be able to play three or four consecutive chord tones, you would have to stretch BIG TIME. In order for us guitar players to be able to play chord voicings, we can move one or two notes to a different register or octave, and the voicing is suddenly much easier to play. This is called a drop voicing.
Now that we have defined what a drop voicing is, I'd like to point out that there are different types of drop voicings. When you have a close voicing and you drop the second note from the top, you have a drop 2 voicing. When you have a close voicing and you drop the third not from the top, you have a drop 3 voicing.