This line is not limited to rappers, as the “rhyme” can represent anyone’s words and feelings. I just happen to be a rapper, so I let out these words and feelings in the form of rhyme.
litmus paper
noun
paper stained with litmus, used to indicate the acidity or alkalinity of a substance. Blue litmus paper turns red under acid conditions, and red litmus paper turns blue under alkaline conditions.
I’m using litmus in a slightly different way, but still as a “decider” of sorts. What you say and think is what determines what sort of prison you will inhabit in your lifetime.