It’s – this is a buildup of x two coming from the decaying of x one or – actually, I said it wrong. It’s a buildup of species b, because that’s a byproduct of the decay of species one, and this is actually then – is actually the decay of x two, of – well, whatever it is, it’s the decay of x two because some of species b is turning into species c. And the final one is this, which is x three dot equals k two times x two, meaning that species c, here, only comes from the decay of species two. That’s this, and this – that’s this bottom row. Okay? ...

See the whole transcript at Introduction to Linear Dynamical Systems - Lecture 10