Negative potential of the oxidation of iron(II) to iron(III)
As IanB2016 mentioned in his answer, the redox potential of a half-cell reaction is always in reference to another half cell.  The usual "reference" half cell in electrochemistry and thermodynamics is the standard hydrogen electrode (SHE).  Some key features of the SHE:


the half cell reaction is $\ce{2 H+(aq) + 2e- &lt;=&gt; H2}$, i.e. the reductant is hydrogen gas and the oxidant is aqueous protons.
the pressure of hydrogen gas for a standard hydrogen electrode is 1 atmosphere.
the concentration of acid in the aqueous electrolyte is 1 molar, which corresponds to a pH of 0.


The oxidation potential of a species is related to its strength as a reducing agent.  Things that have a negative oxidation potential are less reducing than hydrogen gas, and things that have a positive oxidation potential are more reducing than hydrogen gas.  Lithium metal, for example, has an oxidation potential of more than three volts, i.e. $\gt+3\;\mathrm{V}$.  It is an extremely powerful reducing agent, far more powerful than hydrogen gas.

As you note, $\ce{Fe^{2+}}$ has an oxidation potential of $-0.77\;\mathrm{V}$, meaning it is a less powerful reducing agent than hydrogen gas.  This makes sense in several ways:


aqueous solutions of iron(II) salts do not spontaneously reduce aqueous protons to hydrogen gas.  If you mix iron(II) sulfate with water, for example, hydrogen bubbles do not spontaneously form.
Hydrogen gas releases more energy when burned in oxygen than iron(II) salts do when they react with oxygen.  Hydrogen reacts explosively, whereas iron(II) salts do not spontaneously explode or catch fire when they are oxidized by oxygen.  


Finally, note that the oxidation potential of water (via the reaction $\ce{O2(g) + 4 H+ + 4 e− &lt;=&gt; 2H2O}$ at pH 0 at 1 atm of pressure) is -1.229 volts.  Water is a very weak reducing agent, which means that oxygen is a strong oxidizing agent, stronger than iron(III).  This means:


iron(III) salts do not spontaneously oxidize water to produce oxygen.
conversely, oxygen does spontaneously oxidize iron(II) to iron(III).  Solutiosn of iron(II) salts are thermodynamically unstable under aerobic (oxygen-rich) conditions.