Volgens yahoo answers:
"means that the intro, which is usually soft and lacking base, has a really good pause/entrance into the rest of the song. the drop is when there's a big buildup in the intro (in the treble mostly) then a sudden entrance into the rest of the song (with lots of heavy bass)
heavy just means its really intense. half of what determines a good dubstep song is the drop -- if the drop is shitty the song is boring."
Oftewel het punt waar de dikke bass inslaat.