Wellness is a combination of 'well being'- feeling good - and ' fitness'- active with your body.
As mentioned Wellness is a lifestyle that you may want to follow.
A good example of a wellness lifestyle is mindfulness. Mindfulness is paying attention to what happens from moment to moment without judging. For many people, it is one of the most liberating insight; that our thoughts are not 'reality', and that we do not have to identify with them. These are just thoughts.
Why do so many people feel better? In the first place, it is an exercise in order to be more in the here and now. In everyday life we are with our thoughts often in the past or the future. With the attention to be fully present with what is happening now, people learn to overcome their negative thought patterns, and to accept their emotions. Many have experienced that the quality of life increases. We are less in our head, worrying less, live more consciously, at see the beauty of the small things. For example try during a meal to taste the first bite conscious. It might be difficult at the beginning, as we usually eat on autopilot. But once we succeed, it will give us much more appreciation of the beauty of small things.