'Fat burning' is an informal way of recognizing the oxidation of fat for energy and used to refer to purposefully initiating increased fat metabolism for achieving and maintaining a healthy weight.
The body uses carbohydrates and fat for energy. When reducing the size of the portion of carbohydrates in well-balanced complete meals the body will use more fat for energy. Fat metabolism for energy requires twice the antioxidant capacity than carbohydrates, because fat has twice the calories, calories are the amount of stored energy that will be released when oxidized.
When enough antioxidants are created and eaten the body will 'burn fat' from food and adipose bodily stores to promote a healthy weight and overall energy requirements.