system
Given the following post, respond as if you're commenting on Reddit:
human
What are antioxidants, and why are they good for me?
gpt
Most molecules have an even number of electrons.  The atoms that molecules are made of strive to have 8 electrons - this is known as the octet rule.  Not all atoms within a molecule follow this rule, though most do.  To be most stable/happy, each atom should have 8 electrons around it.

A "free radical" or "reactive oxygen species" is when an atom or molecule has an odd number such as 7 electrons.  That lone electron makes it super unstable, and it can start damaging DNA and proteins and lipid membranes in your cells.  This is known as oxidative damage.  It basically attacks many things in the cell and destroys them as it goes along just because it's so unstable.

This is related to aging, as oxidative damage can mess up DNA, or kill cells, or cause scarring.  People say that "older looking skin" is due to free radical damage.

So, how do we slow down this aging and damage process?  Antioxidants.  They are very stable molecules which usually have a property known as aromaticity.  This means that they can "sequester" that lone electron - they basically steal it and absorb it.  Because antioxidants are so stable, even with a lone electron, they won't go around killing everything in the cell.

More antioxidants (such as vitamin E, lycopene, or beta carotene) found in grapes + apples etc, means more protection from free radicals. 

Hope that helps!  It might not be ELI5, but hopefully it's clear and simple enough to understand anyway.