An oxidant is a chemical that can oxidize molecules, which means to remove electrons from molecules. Molecules that are missing electrons are highly reactive and are called free radicals. Free radicals are dangerous to cells because they can react chemically with important biological molecules and cause cell damage. For example, free radicals can chemically react with the DNA in cells. This can lead to mutations in the genetic sequence. These mutations could eventually lead to cancer.
The body has some natural defenses again oxidative damage. These defense molecules are called antioxidants. Some that you may be familiar with are vitamin E and vitamin C. These molecules defend the body's cells from oxidative damage by reacting with the dangerous free radicals and converting them to safer chemicals.