Walt: The reaction has begun.
In the literal sense, Walt means that the chemicals in the cook have started reacting with each other and the batch is almost done. But when viewed in the context of the entire series, it takes on deeper meaning. This is the point of no return for Walt; he has successfully cooked his first batch of crystal meth. Everything from here on out will be a reaction to this pivotal moment.
To help improve the meaning of these lyrics, visit “Ozymandias” by Breaking Bad (Ft. Moira Walley-Beckett & Rian Johnson) and leave a comment on the lyrics box