Preening includes anything done to maintain our physical appearance such as fixing our hair, washing our bodies, wearing clean, matching clothing, brushing our teeth and so forth. In an evolutionary context, preening signifies that we are healthy and in a good state of mind. Have you ever thought about why birds usually have white under-feathers especially around their rear ends? It has been speculated that it is a health advertisement display, and that the carrier of such a feature, have good genetics. Unhealthy animals of any species will usually show symptoms of their sickness with some form of intestinal looseness or diarrhea which would surely show against a wide backdrop! Birds therefore show their health by keeping a clean rear end. They also preen by bathing in water and dust to keep themselves free of ticks and other nuisance hitchhikers.
Humans are no different than any other animal, and like them, we also require a sufficient surplus of energy to maintain healthy bodies. When people fix a collar, they are trying to look good. If they preen someone else, they fancy and are caring for them. If a girl tosses her hair, she is flirting or generally trying to look presentable. Sometimes people preen when nervous, or excited to burn off energy.