Free Radicals are Highly Reactive
Normally, electrons in an atom come in pairs. If an electron is lost, leaving an unpaired electron, the atom becomes reactive. It will seek to stabilize itself by 'stealing' an electron from another atom.
Here's a visual representation of a free radical 'stealing' an electron from a nearby stable atom. The depiction of the atoms is not accurate as many theories have come up of the way electrons move around the nucleus of an atom.
The atom on the left is a stable atom while the atom on the right is a free radical which has a missing pair of electron.
When it comes in contact with a stable atom, it 'steals' an electron from that atom in order to stabilize itself.
The atom on the right now becomes stable while the atom on the left becomes a free radical which is highly reactive and will seek to stabilize itself by 'stealing' an electron from a nearby stable atom. This creates a chain reaction and it occurs very fast.
Antioxidants Terminates this Chain Reaction
When a free radical 'steals' an electron from an antioxidant, the antioxidant does not become reactive. It's their nature to be stable even when an electron is lost from their structure.
As you can see, the free radical chain reaction would go on indefinitely if their are no antioxidants present. Our body is equipped with the necessary antioxidants if the conditions are normal. But because of several factors, our body cannot cope up with the antioxidant requirements. Some factors include pollution, poor eating habits and irregular heavy exercises.