Qi Gong.
Qi - energy
Gong - cultivation
Cultivation of energy.
It is very simple, yet the hardest thing to do right. If you practice on your own, there is a danger of falling into the traps of your own mind: the prejudices of what is right and good Qi and what Qi Gong should be like. A teacher can pinpoint what you are doing wrong, can make you experience the mistakes you make, while without him or her you don't notice them. S/he teaches you to recognize your own mistakes and helps you gain the power to eventually do it by yourself.
"All you need to do is listen to your body, if you are doing something that might damage the body then your body will let you know, wouldn't it?"
Correct. BUT. If you are not used to listening to your body, you might misinterpret what it says to you. Exercises that are heavy can be interpreted as wrong, which results in only doing light exercises which results in learning nothing. Exercises that are painful because of stress in your body can be evaded easily so that you end up just making your own patterns stronger instead of growing. Alternatively you can persevere for too long and increase the stress so much that you get serious lasting injuries. Since many people are misinterpreting their own bodily message system for large periods of time each day, it is, for most people, impossible to learn anything without a teacher. Some people can, but it is an exception to the rule.
A good teacher usually lets you do your own training, and corrects you when you are open to it and ready to learn something. He doesn't enforce his will or his worldview on you. He may seem to make it hard on you, but that is only to help you learn to relax in a new way, to center you in your self.
As my teacher says when somebody is clearly having trouble being in a certain posture, sweating, hurting:
"Don't worry, it only hurts for the first 5 years."
The person usually smiles and relaxes, which makes it easier for a while, OR he just goes on the way he does and hurts himself until he sees that there is no point in straining yourself OR he drops out of the class and starts doing yoga.