The unsaponifiable part of sunflower oil. It's the small part of the oil that resists saponification, the chemical reaction that happens during soap making.
If you want to understand saponification more, here is a short explanation (if not, we understand, just skip this paragraph): Oils are mostly made up of triglyceride molecules (a glycerin + three fatty acids attached to it) and during the soap making process a strong base splits the triglyceride molecule up to become a separate glycerin and three soap molecules (sodium salts of fatty acids). The fantastic Labmuffin blog has a handy explanation with great drawings about the soap-making reaction.
So, the triglyceride molecules are the saponifiable part of the oil, and the rest is the unsaponifiable part. In the case of sunflower oil, it's about 1.5-2% of the oil and consists of skin nourishing molecules like free fatty acids (fatty acids not bound up in a triglyceride molecule, it contains mainly (48-74% according to its spec) barrier building linoleic acid), tocopherol (vitamin E) and sterols.
According to manufacturer's info, it's an oily ingredient that not only simply moisturizes the skin but also has great lipid-replenishing and soothing properties. The clinical study done by the manufacturer (on 20 people) found that a cream with 2% active increases skin moisturization by 48.6% after 1 hour, and 34.2% after 24 hours. Applied twice daily for 4 weeks, the study participants had a major improvement in skin dryness, roughness, and desquamation (skin peeling) parameters.