What occurs when an iron nucleus accepts electrons?
Oxidation is formally the loss of electrons; whereas reduction is formally the gain of those electrons by another species.
You know the old doggerel:
In any chemical reaction, mass AND charge are conserved.
Are charge and mass conserved here? How do you know?
Chemists habitually break these reactions up into half-equations,
Electrons have been introduced into the equation as virtual particles, and when the oxidation and reduction are combined (here they are added 1:1), the electrons cancel:
Why do chemists do this? Well, (i) they wish to torment students, and (ii) for complicated redox reactions it is an efficient and systematic way of balancing equations.
Fe2+ is reduced.
To tell if something is reduced, look at the charge on chemical the left and right hand side. If the charge goes down (like Fe in your question, it goes from +2 on the left to 0 on the right), then it has been reduced, because it has gained electrons (electrons have a -1 charge). If the charge goes up, then it has lost electrons and has been oxidised.
A way to remember it is OILRIG, which stands for:
Loss (of electrons, so the charge goes up in the reaction)
Gain (of electrons, so the charge goes down in the reaction)