Evening primrose, a small yellow wildflower (Oenothera biennis), has been used medicinally for centuries. The oil, pressed from the seed, is rich in linoleic acid, an essential fatty acid (EFA). "Essential" means that we have to consume the nutrient because the body does not produce it.
Fortunately, EFAs are plentiful in foods, notably nuts and seeds. These polyunsaturated fats are good fats, unlike the saturated fats that are bad. Evening primrose oil also supplies another fatty acid, known as gamma-linolenic acid, GLA (not to be confused with alpha-linolenic acid, which is one of the omega-3 fatty acids).