Passage from my textbook:
Are reactions possible among any of the solution components; is so, what is their stoichiometry?
Suppose that you are asked to calculate $[\ce{OH-}]$ in a solution that is prepared to be $\pu{0.10 M}~\ce{NaOH}$ and $\pu{0.20 M}~\ce{NH4Cl}$. Before you answer that $[\ce{OH-}]=\pu{0.10M}$, consider whether a solution can be simultaneously $\pu{0.10M}$ in $\ce{OH-}$ and $\pu{0.20M}$ in $\ce{NH4+}$ . It cannot; any solution containing both $\ce{NH4+}$ and $\ce{OH-}$ must also contain $\ce{NH3}$. The $\ce{OH-}$ and $\ce{NH4+}$ react in a $1:1$ mole ratio until $\ce{OH-}$ is almost totally consumed:$$\ce{NH4+ +OH- ->NH3 + H2O}$$ You are now dealing with the buffer solution $\pu{0.10M}~\ce{NH3}-\pu{0.10M}~\ce{NH4+}$
I am having trouble understanding the third last sentence in the paragraph that starts with "2." The statement reads: "It cannot; any solution containing both $\ce{NH4+}$ and $\ce{OH-}$ must also contain $\ce{NH3}$. The $\ce{OH-}$ and $\ce{NH4+}$ react in $1:1$ mole ratio until $\ce{OH-}$ is almost totally consumed"
Can someone please help me out with understanding this passage?