What is Stoichiometric mixture?

Example 27.11 (Burning methane cleanly)

For $\ce{CH4 + 2O2 -> CO2 + 2H2O}$, $2.0\,\mathrm{mol}$ of methane need $4.0\,\mathrm{mol}$ of dioxygen: $2.0/1 = 4.0/2$. With only $3.0\,\mathrm{mol}$ of dioxygen the mixture of the first example was short of oxygen; in a real flame, that shortage is what makes the poisonous carbon monoxide (Chapter 14).