What does "oxidized form" and "reduced form" mean? What does the phrase "oxidized and reduced forms of the reactants" mean? 

For some context: The book was talking about half-reactions in a half-cell of a galvanic cell, and it said this, 


  Sometimes, both oxidized and reduced forms of the reactants in a half-cell are soluble and cannot be used as an electrode.


I am confused as to what the "oxidized and reduced forms of the reactants in a half-cell" might mean because in a half-cell either only oxidation or reduction takes place.
In this case, the adjective "reduced" does not imply something is actually being reduced. It simply refers to the form of the element in question which has the lower oxidation state. Let's say something is getting oxidised in a half-reaction, for example

$$\ce{Fe^2+ -&gt; Fe^3+ + e-}$$

Iron takes two forms here, $\ce{Fe^2+}$ and $\ce{Fe^3+}$. $\ce{Fe^3+}$ is the oxidised form, that much is obvious, right? Then, $\ce{Fe^2+}$ is the reduced form.

No $\ce{Fe^3+}$ is actually being reduced to $\ce{Fe^2+}$. However, it is still correct to refer to $\ce{Fe^2+}$ as the "reduced form", since it is the form of iron with the lower oxidation state.