When you are born you get what is called Prenatal Qi from your parents. You can think of this as your genetic makeup. This is the base material from which you have to work with in life. This can not be changed. This includes things like physical characteristics- height, bone structure, the state of your organs (think about what is called genetic defects), etc. We also get what is sometimes called our constitution. This is the base from what you have to work with in life.
After we are born we are nourished by what is called Postheaven Qi. Simply put, after we are born we get energy and nourishment from two main sources: the air we breath and the food we eat. The air we breath and the food we eat keeps us alive and healthy. These sources give us the nutrients and the building blocks that we need to repair ourselves and maintain our health and well being.
So what this means is that we are born with a certain constitution and genetic makeup and throughout our life we can keep ourselves healthy and at our optimal state through three different things: diet, exercise, and lifestyle. All three of these things can either enhance our energy or deplete it. Qigong falls under the category of exercise. And if you notice, that is only one of the three. In other words Qigong can be a big part of keeping yourself healthy and functioning at an optimal state but it is only part of the picture. Other parts of your life are also important- namely diet and lifestyle.
The practice of Qigong helps to keep the energy in the body flowing freely so that every part of the body can get the energy and the nutrients that it needs to maintain itself, repair, and grow stronger.