The working principle of oxidation-reduction works in the same way of the first known batteries, more than two centuries ago. As it happens with the lemon, it's a very interesting example to learn how batteries work.
To achieve voltage from this fruit the user should first get two metal plates, one from copper and the other zinc based. Perhaps someone would ask why not two copper plates, and the answer lies on positive and negative elements. The copper electrode is the positive terminal of the battery and the zinc the negative one. Going a bit deeper on this, copper is less electronegative than zinc so it loses electrons and becomes positive so that means a "+" in the battery. Electrons flow then from the minus (zinc) terminal to the positive (copper).
The lemon juice acts in this improvised battery as the electrolyte for the oxidation-reduction chemical reaction. The juice provides electrons to travel around the circuit, so if you take an apparatus that measures voltage, it will show a reading and therefore also electric current!
Several lemons are enough to power a LED and create light or also for other circuits that low requirements. Is this the secret of the electrifying taste of lemons?