Oxidation occurs when oxygen reacts with another substance. Rust is oxygen reacting with iron. A candle burning is oxygen reacting with hydrocarbon compounds releasing CO2, water and heat. Your body cannot function without oxidation happening in your cells. Oxygen gives fire to the chemical reactions which then help the cell function properly.
But if there are too many free radicals being produced and not enough antioxidants to neutralise them, this can lead to oxidative stress. Continued oxidative stress can lead to oxidative damage and chronic inflammation. That means cell damage, cell mutation and more seriously, cancer, diabetes, cardiovascular, neurological and pulmonary diseases.