An 808 is a signature bass (in this context. 808 refers to the Roland TR-808, one of the first drum synthesizers, and a large part of southern hip hop sound). The bass drop is often a signal that the beat is starting, so it is go time.
From a literal standpoint, 808 pounds is a lot, so why would you be carrying it?
Note the double syllable rhyme scheme (with assonance):
Drop rhymes with lock It rhymes with et
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