Nas' raps should be locked in a cell,
1) Nas' raps are so powerful and dangerous that they have to be controlled, thus likening them to a criminal
2) His rhymes are so important that to ensure they are never lost or forgotten, they must be stored away in a secure way
This line asserts that prisons don’t hold that which is unfit for society, but that they hold that which is too powerful for it. In this sense, he claims that incarceration is not the weak being reformed by society, but the powerful being oppressed. Likewise, it means his rhymes are feared by society and defy it in the same way his incarcerated comrades have.
Note that this is the last bar of the entire album. The first bar (of rhyming) is “yeah, straight out the fuckin' dungeons of rap/ where fake niggas don’t make it back.” This structure, with the last line leading directly back to the first one, gives the album a kind of Finnegans Wake looping structure.
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