How to find the fidelity between two state when one is an operator?

I am going through Nielsen and Chuang and am finding the chapter on error-correction particularly confusing. At the moment I am stuck on exercise 10.12 which states

Show that the fidelity between the state $|0 rangle$ and $varepsilon(|0ranglelangle0|)$ is $sqrt{1-2pbackslash3}$, and use this to argue that the minimum fidelity for the depolarizing channel is $sqrt{1-2pbackslash3}$.

As I understand $varepsilon$ is a quantum operation and could be whatever we want as long as it fits the definition, do I assume $varepsilon$ is the depolarizing channel or is there some general operation I don’t know about.

Thanks!