Oxidation of Vitamin C
Think of oxidation in a classical sense for a minute or so. Oxidation is the addition of oxygen to a molecule or it can also mean the removal of hydrogen. If you carefully look at the structure of ascorbic acid, you will see that the two -$\ce{OH}$ groups are now converted into carbonyl groups. Those two hydrogens can be lost as a proton, $\ce{H+}$.