The primary function of anti-oxidants are to protect against Oxidative Stress.
Oxidative Stress is a state of imbalance between Reactive Oxygen Species (ROS) and natural protections within the cell.
ROS are formed within the mitochodria and external to the cell by reaction of unstable molecules. ROS are vital to many important cellular signalling functions (REDOX signalling).
Your cells have natural mechanisms for reducing ROS such as SuperOxide DIsmutase, Glutathione Peroxidase and Catylase. When these systems become overwhelmed by excess ROS, a state of Oxidative Stress occurs.
Think of Oxidative Stress as a state of accelerated cellular rusting.

Enzymes are the most powerful class of
anti-oxidants.
The most powerful anti-oxidants, Super Oxide
Dismutase, Catylase and Gutathione
Peroxidase are all enzymes.