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From Wikipedia:

Pourbaix Diagram of aqueous Iron

What does this graph mean?

What is $\varepsilon$? It looks like $\varepsilon_0$, like the vacuum permittivity, but what does this have to do with the polymorph and/or isomers of ferrite or hematite?

And what is the V or 'five' in square brackets after the $\varepsilon_0$?

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This diagram, called E-pH diagram or Pourbaix diagram, makes it possible to know the domains of existence or of predominance of redox species as a function of pH.

The abscissa is the pH, the ordinate the reduction potential (with respect to the standard hydrogen electrode): thus, it is not a question of the size epsilon but of the potential $E$ which has been a little "stylized" as $\mathcal{E}$, which is expressed in volts (hence the symbol V).

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This is a Pourbaix Diagram (I assume this is where you got this figure). While it's not a common notation as far as I'm aware, $\epsilon_0$ is being used to represent the reduction potential in volts (V). As mentioned at the top of the Wikipedia page, this is basically a phase diagram, that tells you the ranges of voltage/pH for which a particular species of (in this case) iron is most thermodynamically favorable. The dividing lines coincide with an even mixture of the surrounding species.

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    $\begingroup$ It's probably meant to be $\mathcal{E}_0$ $\mathcal{E}_0$, as in electromotive force. $\endgroup$
    – orthocresol
    Feb 13 at 2:14

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