Oxidation and reduction reactions are really all about the transfer of electrons.
In biology, this transfer of electrons can be identified by watching what happens to the H.

If an organic molecule gains H, it has beenreduced.
If an organic molecule loses H, it has been oxidized. 

Glucose contains H. When glucose is broken down during cellular respiration,
  these hydrogen are removed from the glucose and the energy from breaking the bonds with hydrogen is used to make ATP.

 Glucose is oxidized.

What happens to these hydrogen atoms (and their electrons)?
The oxygen we breathe in eventually bonds to these hydrogen atoms to get them out of the cell.

Oxygen is reduced.
​ 
At its simplest, we breathe in oxygen to get rid of the hydrogen left over from the
​ breakdown of glucose.

During the breakdown of glucose, energy is released. The electrons "carry" this energy
from the bonds of glucose to a lower-energy state bonded to oxygen. 
The energy that's released as these electrons move to a lower-energy state can be captured.
In cellular respiration, electrons from glucose move gradually through the electron transport chain towards oxygen, passing to lower and lower energy states and releasing energy at each step. The goal of cellular respiration is to capture this energy in the form of ATP.

Oxidation/Reduction reactions

=
​REDOX reactions

these are the electron carriers in the electron transport chain (ETC)

reduction potential

energy associated with bonding to  electrons and their H