Redox is the abbreviated term for reduction-oxidation.  Coenzyme Q10 molecules are redox molecules.   Redox refers to the oxidation state of the molecule.  Oxidized Coenzyme Q10 molecules, called ubiquinone (CoQ10), are Coenzyme Q10 molecules that can accept (take on) two electrons and thus become reduced Coenzyme Q10 molecules called ubiquinol.

Reduced Coenzyme Q10 molecules, called ubiquinol (CoQH2), are Coenzyme Q10 molecules that can donate (lose or give up) two electrons and thus become oxidized Coenzyme Q10 molecules called ubiquinone.

The highly unstable intermediate state of Coenzyme Q10 molecules is the ubisemiquinone molecule, which has lost one electron and stands to lose the second electron.  It is the redox nature of the Coenzyme Q10 molecules that make them valuable both as components of cellular bio-energetics and antioxidant defense.

Redox reactions take place simultaneously.  One atom or molecule donates an electron (becomes oxidized), and, at the same time, another atom or molecule accepts the electron (becomes reduced).