The drop is the point in a track where a switch of rhythm or bass line occurs and usually follows a recognizable build section and break.
In Hip-Hop and electronic music, the reintroduction of the full bass line and drums is known as the drop.
In Dubstep, the drop involves a full bass line and commonly a "wobble" bass or "vowel" bass accompanied by a strong shuffling beat.
In Metalcore sub-genres, bass drops are often utilized under the first chord of a breakdown, to emphasize the breakdown and give it a pronounced presence.You can expect it when the music goes silent then hear a loud "boom".
In drum and bass, skilled DJs sometimes perform what is called the "double drop": beat matching two tracks in a way that the drop, and hence the respective climaxes, occur at the same time.