What does the "ii" in the name iron(ii) indicate about the cation?

It is indicating the oxidation state of Iron.

Explanation:

Oxidation state indicates the degree of oxidation or the charge of an element when it is forming a molecule or when it is in a ionic form (anion or cation). In the case of Iron (ii) or Fe(ii), it means that the oxidation state is 2 and as Iron form cations, its oxidation state is possitive, it means +2. In this state, Iron tend to combine with an anion (negative charge), forming a compound.