The question in the title is a very important one, and I think all of us should have this question in mind and search for a complete answer. The truth is though, that we only ask about health when we have a serious problem. A official definition of health is given by The World Health Organization who defined health in its broader sense in 1946 as "a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity". For 1946 it was not a bad definition, but let's face it, times have changed and life has evolved since then, so today we have 2 new aspects of health to consider: emotional health and spiritual health. If we take a step back and try to see the "big picture" we realize that each aspect of health influences the other aspects, for example if someone has an emotional problem it will cause physical problems, spiritual problems, mental problems and social problems too, so it's very important to solve the problem when we first see it even if we don't think that an emotional problem is important. In the incipient phase the emotional problem could be solved by simply talking to a friend, but if we let it develop it could need serious professional help. After this example I think you will want to know more about each aspect of health, so let's see what they mean and how can we improve them.
Physical health - the first thing that comes to my mind when I think about perfect physical health are babies - they are the best example because they run all around the place and use their energy until they fall asleep. If we could do the same, I'm sure we would have a better physical health. A definition of physical health would be that it is a state in which all of the organs are functioning well 100% of the time, so in order for that to happen we have to give our body good nutrients, good exercise and enough rest. It may sound too simple, but it shouldn't be more complicated than this.
Mental health - one good example for this is inventor Thomas Alva Edison, he tried and failed more thousand times to invent the light bulb, but he was strong enough mentally not to give up, and he eventually succeeded. Mental health is a state of well-being that allows you to reach your true potential and contribute to the community. It may seem strange for some of you, but actually helping others is the greatest thing that we can do and it keeps us happy and healthy, so in order to improve your mental health start by helping the people around you, I can tell you for sure that you will feel very good by doing so.
Emotional health - imagine a little girl on a Christmas evening opening her presents, that for me is an example of good emotional health, because gratitude and happiness are the healthiest emotions for us. I think emotional health is about feeling good, grateful, happy and not being afraid to share these emotions with all the people we interact with. A good way to improve our social health is by doing both physical and mental exercises that we like to do, for example if you like to play tennis and chess, please do so as often as you can.
Social health - the best indicator for social health is the ability to make friends. The social aspect of health is very important, because we are social beings and it's scientifically proven that if someone is ill and their loved ones are with them, they recover faster than if they were to stay alone. A tip on how to improve your social health would be to understand the fact that "it's nice to be important, but it's more important to be nice".
Spiritual health - is the most important aspect of health from my perspective because it influences all aspects of our health and often it is neglected. We have to take care of our soul as well as we take care of our physical body and the best way to do this is by meditation. Please don't be scared if you don't know what meditation is or how you can do it, because it's a lot easier than you think, in fact you don't have to do anything, just stay still and silent. If you can remain still and silent for 15 minutes a day and gradually increase this interval to an hour a day, it will have a great influence on your spiritual health and on your overall health too.