A free radical is largely a molecule which has an unpaired electron.  It forms as a result of the strategy called oxidation which is just the chemical reaction that takes place when something mixes with oxygen.  

Let’s just take a step backwards to basic chemistry if this is all rather too confusing.  

The body contains many billions of cells.  The cells are of many assorted types depending on where they are found in the body.  In the cells are compounds called molecules which are a combination of the smaller building block, atoms.  

Inside atoms are a nucleus ( containing protons and neutrons ) and electrons.  The electrons are adversely charged particles that circle the central nucleus of the atom.  

The electrons enjoy going in pairs but during the process of oxidation one electron is lost leaving a lonesome, single, unpaired electron.  Any molecule or atom containing an unpaired electron is called a free radical.  

Free radicals don’t love to have a lonesome electron so they steal one from the closest stable atom setting off a reaction.  

the consequences of free radical formation inside cells causes defects