If you studied chemistry at school or later, you probably ended up with the idea that chemical reactions are always neatly balanced. In other words, a particular reaction begins with a set of complete chemicals on one side of the equation, and the reaction if there is one, ends up with another set of complete and tidy chemicals on the other side, with no bits left over in some sort of untidy manner.
What has been discovered in the last fifty or so years, especially in biology, is that this is not so. During reactions that occur during the functioning of our bodies, for example, all over the place bits are left over from reactions; millions of anions ( -ve ions, with excess electrons ) and cations ( +ve ions, lacking electrons ).
The cations in particular cause damage to our bodies. When created they find a way to rob some part (usually a living cell) of an electron, and thus inflict severe damage. In the body, these cations are called free radicals.
Free radicals come not only from food we eat, but from UV light, chemicals in the air, and so on.
A natural defence against the damage caused is through the use of anti-oxidants.
In Dr Colgan's book 'Optimal Sports Nutrition - A Complete Nutritional Guide for Optimising Athletic Performance', he discusses free radicals and anti-oxidants.
Another source of information is the small book put out in conjunction with Enzogenol, which is:
Dr Kelvin Duncan (Ed.) 1998.Fighting Free Radicals; The Enzogenol TM Story. The Pacific Scientific Press. pp 88. (Christchurch, Auckland) ISBN 0 473 05349 7.
Both books list plenty of scientific, peer-reviewed literature, for you to read to get further background information. There are also some web-sites listed, but for the hard-stuff, get paper-based information.